refactor(backend): migrate db.DB to db.Conn PoolProxy across all handlers

Replace direct *pgxpool.Pool usage with PoolProxy wrapper across the entire backend:

- db.DB renamed to db.Conn (*pgxpool.Pool -> *PoolProxy)
- JWT functions now accept context.Context instead of using context.Background()
- Handler DB calls route through PoolProxy for per-test transaction support
- Fixture/helper/testdb functions accept Querier interface for decoupling
- Query ordering fixed in bookings handlers: COUNT after data query to avoid pgx conn busy
- Time truncation fixed: time.Date instead of Truncate(24*time.Hour) for week start calc
- testmain_test.go files updated with SeedBaseline and NewPoolProxy

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

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

backend/handlers/notifications/notifications.go

@@ -120,8 +120,15 @@ func GetNotifications(w http.ResponseWriter, r *http.Request) {
 	baseQuery += fmt.Sprintf(" LIMIT $%d", param)
 	args = append(args, perPage+1)
 
+	var total int
+	countWhere := ""
+	if !includeAcknowledged {
+		countWhere += " WHERE an.acknowledged_at IS NULL"
+	}
+	db.Conn.QueryRow(r.Context(), "SELECT COUNT(*) FROM admin_notifications an"+countWhere).Scan(&total)
+
 	// Query
-	rows, err := db.DB.Query(r.Context(), baseQuery, args...)
+	rows, err := db.Conn.Query(r.Context(), baseQuery, args...)
 	if err != nil {
 		log.Printf("Failed to fetch notifications: %v", err)
 		http.Error(w, "Internal server error", http.StatusInternalServerError)
@@ -129,13 +136,6 @@ func GetNotifications(w http.ResponseWriter, r *http.Request) {
 	}
 	defer rows.Close()
 
-	var total int
-	countWhere := ""
-	if !includeAcknowledged {
-		countWhere += " WHERE an.acknowledged_at IS NULL"
-	}
-	db.DB.QueryRow(r.Context(), "SELECT COUNT(*) FROM admin_notifications an"+countWhere).Scan(&total)
-
 	var notifications []AdminNotification
 
 	for rows.Next() {
@@ -208,7 +208,7 @@ func GetNotifications(w http.ResponseWriter, r *http.Request) {
 // Returns the count of unacknowledged notifications for the bell icon.
 func GetUnreadCount(w http.ResponseWriter, r *http.Request) {
 	var count int
-	err := db.DB.QueryRow(r.Context(),
+	err := db.Conn.QueryRow(r.Context(),
 		`SELECT COUNT(*) FROM admin_notifications WHERE acknowledged_at IS NULL`,
 	).Scan(&count)
 	if err != nil {
@@ -238,7 +238,7 @@ func AcknowledgeNotification(w http.ResponseWriter, r *http.Request) {
 		WHERE id = $1 AND acknowledged_at IS NULL
 	`
 
-	cmdTag, err := db.DB.Exec(r.Context(), query, idStr)
+	cmdTag, err := db.Conn.Exec(r.Context(), query, idStr)
 	if err != nil {
 		log.Printf("Failed to acknowledge notification %s: %v", idStr, err)
 		http.Error(w, "Internal server error", http.StatusInternalServerError)