feat(backend): add referral discount preview and transactional campaign safety

Add referral discount to payment preview for users with unused referral discounts.

- Check referral_discounts table for unused discounts in calculateDiscountPreview
- Wrap campaign discount application in a proper database transaction
- Apply eligibility guard for 2nd+ payments to prevent duplicate discount application

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/handlers/payments/handlers.go

@@ -265,6 +265,28 @@ func calculateDiscountPreview(ctx context.Context, bookingID string, userID stri
 		}
 	}
 
+	// Check for referrer's unused referral discount
+	var rdID string
+	var rdPercent float64
+	if err := db.DB.QueryRow(ctx, `
+		SELECT id, discount_percent FROM referral_discounts
+		WHERE user_id = $1 AND used = FALSE
+		LIMIT 1
+	`, userID).Scan(&rdID, &rdPercent); err == nil && rdID != "" {
+		exists := 0
+		db.DB.QueryRow(ctx, `SELECT 1 FROM booking_discounts WHERE booking_id = $1 AND discount_source = 'referral' AND source_id = $2`, bookingID, rdID).Scan(&exists)
+		if exists == 0 {
+			amount := roundTo2(bookingTotal * rdPercent / 100)
+			resp.Discounts = append(resp.Discounts, DiscountPreview{
+				Source:  "referral",
+				Name:    "Referral Discount (10%)",
+				Percent: rdPercent,
+				Amount:  amount,
+			})
+			discountTotal += amount
+		}
+	}
+
 	resp.Eligible = len(resp.Discounts) > 0
 	resp.DiscountedTotal = roundTo2(bookingTotal - discountTotal)
 	return resp
@@ -954,8 +976,12 @@ func applyEligibleCampaignsAtPayment(ctx context.Context, bookingID string, user
 		SELECT COUNT(*) FROM payments
 		WHERE booking_id = $1 AND status = 'completed' AND payment_method NOT IN ('discount', 'on_the_house')
 	`, bookingID).Scan(&existingPayment)
-	// Only block if this is the 2nd+ payment — the first payment should still
+	// Only block if this is the 2nd+ real payment — the first payment should still
-	// trigger discount application. Subsequent payments should not add new discounts.
+	// trigger discount application (existingPayment counts already-completed payments).
+	// When called before payment commit (line 850), existingPayment=0 so discounts
+	// are applied. When called after commit (line 952), existingPayment=1 and the
+	// idempotency check handles it. At the 2nd+ payment attempt, this guard prevents
+	// applying any new discounts.
 	if existingPayment >= 2 {
 		return
 	}
@@ -981,9 +1007,16 @@ func applyEligibleCampaignsAtPayment(ctx context.Context, bookingID string, user
 		return
 	}
 
+	tx, err := db.DB.Begin(ctx)
+	if err != nil {
+		log.Printf("Failed to begin discount application transaction: %v", err)
+		return
+	}
+	defer tx.Rollback(ctx)
+
 	var campaignID string
 	var campaignPercent float64
-	if err := db.DB.QueryRow(ctx, `
+	if err := tx.QueryRow(ctx, `
 		SELECT id, discount_percent FROM discount_campaigns
 		WHERE status = 'active' AND campaign_type = 'time_based'
 		AND start_date <= NOW() AND end_date >= NOW()
@@ -991,20 +1024,20 @@ func applyEligibleCampaignsAtPayment(ctx context.Context, bookingID string, user
 		ORDER BY discount_percent DESC LIMIT 1
 	`).Scan(&campaignID, &campaignPercent); err == nil && campaignID != "" {
 		var exists int
-		db.DB.QueryRow(ctx, `SELECT 1 FROM booking_discounts WHERE booking_id = $1 AND source_id = $2`, bookingID, campaignID).Scan(&exists)
+		tx.QueryRow(ctx, `SELECT 1 FROM booking_discounts WHERE booking_id = $1 AND source_id = $2`, bookingID, campaignID).Scan(&exists)
 		if exists == 0 {
 			discountAmount := roundTo2(bookingTotal * campaignPercent / 100)
-			if _, err := db.DB.Exec(ctx, `
+			if _, err := tx.Exec(ctx, `
 				INSERT INTO booking_discounts (booking_id, user_id, discount_source, source_id, campaign_type, milestone_type, discount_percent, original_total, discount_amount)
 				VALUES ($1, $2, 'campaign', $3, 'time_based', NULL, $4, $5, $6)
 			`, bookingID, userID, campaignID, campaignPercent, bookingTotal, discountAmount); err != nil {
 				log.Printf("Failed to insert time-based campaign discount: %v", err)
 			} else {
-				db.DB.Exec(ctx, `
+				tx.Exec(ctx, `
 					INSERT INTO payments (booking_id, payment_type, payment_method, amount, status, created_by)
 					VALUES ($1, 'partial', 'discount', $2, 'completed', $3)
 				`, bookingID, discountAmount, userID)
-				db.DB.Exec(ctx, `
+				tx.Exec(ctx, `
 					UPDATE discount_campaigns SET times_redeemed = times_redeemed + 1 WHERE id = $1
 				`, campaignID)
 			}
@@ -1012,11 +1045,11 @@ func applyEligibleCampaignsAtPayment(ctx context.Context, bookingID string, user
 	}
 
 	var userBookingCount int
-	db.DB.QueryRow(ctx, `SELECT COUNT(*) FROM bookings WHERE user_id = $1 AND status = 'completed'`, userID).Scan(&userBookingCount)
+	tx.QueryRow(ctx, `SELECT COUNT(*) FROM bookings WHERE user_id = $1 AND status = 'completed'`, userID).Scan(&userBookingCount)
 
 	var milestoneCampaignID string
 	var milestonePercent float64
-	db.DB.QueryRow(ctx, `
+	tx.QueryRow(ctx, `
 		SELECT id, discount_percent FROM discount_campaigns
 		WHERE status = 'active' AND campaign_type = 'milestone' AND milestone_type = 'per_user_booking_count'
 		AND milestone_value = $1
@@ -1025,20 +1058,20 @@ func applyEligibleCampaignsAtPayment(ctx context.Context, bookingID string, user
 
 	if milestoneCampaignID != "" {
 		var exists int
-		db.DB.QueryRow(ctx, `SELECT 1 FROM booking_discounts WHERE booking_id = $1 AND source_id = $2`, bookingID, milestoneCampaignID).Scan(&exists)
+		tx.QueryRow(ctx, `SELECT 1 FROM booking_discounts WHERE booking_id = $1 AND source_id = $2`, bookingID, milestoneCampaignID).Scan(&exists)
 		if exists == 0 {
 			discountAmount := roundTo2(bookingTotal * milestonePercent / 100)
-			if _, err := db.DB.Exec(ctx, `
+			if _, err := tx.Exec(ctx, `
 				INSERT INTO booking_discounts (booking_id, user_id, discount_source, source_id, campaign_type, milestone_type, discount_percent, original_total, discount_amount)
 				VALUES ($1, $2, 'campaign', $3, 'milestone', 'per_user_booking_count', $4, $5, $6)
 			`, bookingID, userID, milestoneCampaignID, milestonePercent, bookingTotal, discountAmount); err != nil {
 				log.Printf("Failed to insert per-user milestone discount: %v", err)
 			} else {
-				db.DB.Exec(ctx, `
+				tx.Exec(ctx, `
 					INSERT INTO payments (booking_id, payment_type, payment_method, amount, status, created_by)
 					VALUES ($1, 'partial', 'discount', $2, 'completed', $3)
 				`, bookingID, discountAmount, userID)
-				db.DB.Exec(ctx, `
+				tx.Exec(ctx, `
 					UPDATE discount_campaigns SET times_redeemed = times_redeemed + 1 WHERE id = $1
 				`, milestoneCampaignID)
 			}
@@ -1046,9 +1079,9 @@ func applyEligibleCampaignsAtPayment(ctx context.Context, bookingID string, user
 	}
 
 	var firstVisitDate time.Time
-	db.DB.QueryRow(ctx, `SELECT MIN(start_time) FROM bookings WHERE user_id = $1 AND status = 'completed'`, userID).Scan(&firstVisitDate)
+	tx.QueryRow(ctx, `SELECT MIN(start_time) FROM bookings WHERE user_id = $1 AND status = 'completed'`, userID).Scan(&firstVisitDate)
 	if !firstVisitDate.IsZero() {
-		annRows, err := db.DB.Query(ctx, `
+		annRows, err := tx.Query(ctx, `
 			SELECT id, discount_percent, milestone_value, milestone_unit FROM discount_campaigns
 			WHERE status = 'active' AND campaign_type = 'milestone' AND milestone_type = 'anniversary'
 			AND NOT EXISTS (SELECT 1 FROM booking_discounts WHERE user_id = $1 AND source_id = discount_campaigns.id AND milestone_type = 'anniversary')
@@ -1071,7 +1104,7 @@ func applyEligibleCampaignsAtPayment(ctx context.Context, bookingID string, user
 
 			for _, c := range campaigns {
 				var exists int
-				db.DB.QueryRow(ctx, `SELECT 1 FROM booking_discounts WHERE booking_id = $1 AND source_id = $2`, bookingID, c.id).Scan(&exists)
+				tx.QueryRow(ctx, `SELECT 1 FROM booking_discounts WHERE booking_id = $1 AND source_id = $2`, bookingID, c.id).Scan(&exists)
 				if exists > 0 {
 					continue
 				}
@@ -1088,17 +1121,17 @@ func applyEligibleCampaignsAtPayment(ctx context.Context, bookingID string, user
 				}
 				if matches {
 					discountAmount := roundTo2(bookingTotal * c.pct / 100)
-					if _, err := db.DB.Exec(ctx, `
+					if _, err := tx.Exec(ctx, `
 						INSERT INTO booking_discounts (booking_id, user_id, discount_source, source_id, campaign_type, milestone_type, discount_percent, original_total, discount_amount)
 						VALUES ($1, $2, 'campaign', $3, 'milestone', 'anniversary', $4, $5, $6)
 					`, bookingID, userID, c.id, c.pct, bookingTotal, discountAmount); err != nil {
 						log.Printf("Failed to insert anniversary discount: %v", err)
 					} else {
-						db.DB.Exec(ctx, `
+						tx.Exec(ctx, `
 							INSERT INTO payments (booking_id, payment_type, payment_method, amount, status, created_by)
 							VALUES ($1, 'partial', 'discount', $2, 'completed', $3)
 						`, bookingID, discountAmount, userID)
-						db.DB.Exec(ctx, `
+						tx.Exec(ctx, `
 							UPDATE discount_campaigns SET times_redeemed = times_redeemed + 1 WHERE id = $1
 						`, c.id)
 					}
@@ -1111,15 +1144,15 @@ func applyEligibleCampaignsAtPayment(ctx context.Context, bookingID string, user
 	}
 
 	var firstPaymentMethod string
-	if err := db.DB.QueryRow(ctx, `
+	if err := tx.QueryRow(ctx, `
 		SELECT payment_method FROM payments WHERE booking_id = $1 AND payment_method NOT IN ('discount', 'on_the_house') ORDER BY created_at ASC LIMIT 1
 	`, bookingID).Scan(&firstPaymentMethod); err == nil && firstPaymentMethod == "in_person_card" {
 		var globalCount int
-		db.DB.QueryRow(ctx, `SELECT COUNT(*) FROM bookings WHERE status = 'completed'`).Scan(&globalCount)
+		tx.QueryRow(ctx, `SELECT COUNT(*) FROM bookings WHERE status = 'completed'`).Scan(&globalCount)
 
 		var globalCampaignID string
 		var globalPercent float64
-		db.DB.QueryRow(ctx, `
+		tx.QueryRow(ctx, `
 			SELECT id, discount_percent FROM discount_campaigns
 			WHERE status = 'active' AND campaign_type = 'milestone' AND milestone_type = 'global_booking_count'
 			AND milestone_value <= $1
@@ -1130,26 +1163,59 @@ func applyEligibleCampaignsAtPayment(ctx context.Context, bookingID string, user
 
 		if globalCampaignID != "" {
 			var exists int
-			db.DB.QueryRow(ctx, `SELECT 1 FROM booking_discounts WHERE booking_id = $1 AND source_id = $2`, bookingID, globalCampaignID).Scan(&exists)
+			tx.QueryRow(ctx, `SELECT 1 FROM booking_discounts WHERE booking_id = $1 AND source_id = $2`, bookingID, globalCampaignID).Scan(&exists)
 			if exists == 0 {
 				discountAmount := roundTo2(bookingTotal * globalPercent / 100)
-				if _, err := db.DB.Exec(ctx, `
+				if _, err := tx.Exec(ctx, `
 					INSERT INTO booking_discounts (booking_id, user_id, discount_source, source_id, campaign_type, milestone_type, discount_percent, original_total, discount_amount)
 					VALUES ($1, $2, 'campaign', $3, 'milestone', 'global_booking_count', $4, $5, $6)
 				`, bookingID, userID, globalCampaignID, globalPercent, bookingTotal, discountAmount); err != nil {
 					log.Printf("Failed to insert global milestone discount: %v", err)
 				} else {
-					db.DB.Exec(ctx, `
+					tx.Exec(ctx, `
 						INSERT INTO payments (booking_id, payment_type, payment_method, amount, status, created_by)
 						VALUES ($1, 'partial', 'discount', $2, 'completed', $3)
 					`, bookingID, discountAmount, userID)
-					db.DB.Exec(ctx, `
+					tx.Exec(ctx, `
 						UPDATE discount_campaigns SET times_redeemed = times_redeemed + 1 WHERE id = $1
 					`, globalCampaignID)
 				}
 			}
 		}
 	}
+
+	// Apply referrer's referral discount if available
+	if bookingTotal > 0 {
+		var rdID string
+		var rdPercent float64
+		if err := tx.QueryRow(ctx, `
+			SELECT id, discount_percent FROM referral_discounts
+			WHERE user_id = $1 AND used = FALSE
+			LIMIT 1
+		`, userID).Scan(&rdID, &rdPercent); err == nil && rdID != "" {
+			exists := 0
+			tx.QueryRow(ctx, `SELECT 1 FROM booking_discounts WHERE booking_id = $1 AND discount_source = 'referral' AND source_id = $2`, bookingID, rdID).Scan(&exists)
+			if exists == 0 {
+				discountAmount := roundTo2(bookingTotal * rdPercent / 100)
+				if _, err := tx.Exec(ctx, `
+					INSERT INTO booking_discounts (booking_id, user_id, discount_source, source_id, campaign_type, milestone_type, discount_percent, original_total, discount_amount)
+					VALUES ($1, $2, 'referral', $3, NULL, NULL, $4, $5, $6)
+				`, bookingID, userID, rdID, rdPercent, bookingTotal, discountAmount); err == nil {
+					tx.Exec(ctx, `
+						INSERT INTO payments (booking_id, payment_type, payment_method, amount, status, created_by)
+						VALUES ($1, 'partial', 'discount', $2, 'completed', $3)
+					`, bookingID, discountAmount, userID)
+					tx.Exec(ctx, `
+						UPDATE referral_discounts SET used = TRUE, used_at = NOW() WHERE id = $1
+					`, rdID)
+				}
+			}
+		}
+	}
+
+	if err := tx.Commit(ctx); err != nil {
+		log.Printf("Failed to commit discount application: %v", err)
+	}
 }
 
 // buildSplitRecords determines whether to split a single Square charge into