diff --git a/backend/internal/jobs/cleanup.go b/backend/internal/jobs/cleanup.go
index 1f6ee6b..a16e9b4 100644
--- a/backend/internal/jobs/cleanup.go
+++ b/backend/internal/jobs/cleanup.go
@@ -1,14 +1,21 @@
 package jobs
 
 import (
+	"context"
+	"errors"
+	"fmt"
+	"log/slog"
 	"time"
 
 	"crussell/auth"
+	"crussell/db"
 	authHandlers "crussell/handlers/auth"
 	"crussell/handlers/payments"
 	"crussell/handlers/scheduling"
 	"crussell/handlers/user"
 	"crussell/mw"
+
+	"github.com/jackc/pgx/v5"
 )
 
 // RegisterAll registers every background maintenance job on the scheduler.
@@ -214,4 +221,41 @@ func RegisterAll(s *Scheduler) {
 		Concurrency: 1,
 		Handler:     scheduling.CleanupExpiredRefreshTokens,
 	})
+
+	// Staggered from cleanup-verification-codes / cleanup-refresh-tokens
+	// (both at 0 2 * * *) to avoid DB contention.
+	s.Register(Job{
+		Name:        "sweep-square-webhook-events",
+		Schedule:    "30 2 * * *", // Daily at 2:30am
+		Timeout:     30 * time.Second,
+		Concurrency: 1,
+		Handler:     SweepSquareWebhookEvents,
+	})
+}
+
+// SweepSquareWebhookEvents deletes square_webhook_events rows older than 90
+// days. Every accepted webhook event_id is stored permanently for restart-safe
+// dedup (payment.updated fires on every payment update), so without retention
+// the table would grow without bound. 90 days comfortably exceeds Square's
+// webhook replay window while keeping the table bounded.
+func SweepSquareWebhookEvents(ctx context.Context) (int, error) {
+	tx, err := db.Conn.Begin(ctx)
+	if err != nil {
+		return 0, fmt.Errorf("failed to begin transaction: %w", err)
+	}
+	defer func() {
+		if err := tx.Rollback(ctx); err != nil && !errors.Is(err, pgx.ErrTxClosed) {
+			slog.Error("failed to rollback transaction", "err", err)
+		}
+	}()
+
+	tag, err := tx.Exec(ctx, `
+		DELETE FROM square_webhook_events
+		WHERE received_at < NOW() - INTERVAL '90 days'
+	`)
+	if err != nil {
+		return 0, fmt.Errorf("failed to sweep square webhook events: %w", err)
+	}
+
+	return int(tag.RowsAffected()), tx.Commit(ctx)
 }
diff --git a/backend/internal/jobs/cleanup_test.go b/backend/internal/jobs/cleanup_test.go
new file mode 100644
index 0000000..1fd3df6
--- /dev/null
+++ b/backend/internal/jobs/cleanup_test.go
@@ -0,0 +1,89 @@
+//go:build test
+
+package jobs
+
+import (
+	"context"
+	"os"
+	"testing"
+
+	"crussell/db"
+	"crussell/testutils/testdb"
+)
+
+func TestMain(m *testing.M) {
+	pool := testdb.CreateTestDatabase("crussell_test_jobs")
+	db.Conn = db.NewPoolProxy(pool)
+	code := m.Run()
+	testdb.DestroyTestDatabase(pool, "crussell_test_jobs")
+	os.Exit(code)
+}
+
+// ============================================================
+// SweepSquareWebhookEvents Tests
+// ============================================================
+
+func TestSweepSquareWebhookEvents_DeletesOldRows(t *testing.T) {
+	ctx := context.Background()
+
+	// Recent event (NOW() — must be preserved)
+	if _, err := db.Conn.Exec(ctx,
+		"INSERT INTO square_webhook_events (event_id, received_at) VALUES ($1, NOW())",
+		"evt_recent"); err != nil {
+		t.Fatalf("failed to insert recent webhook event: %v", err)
+	}
+
+	// Old event (100 days ago — must be swept)
+	if _, err := db.Conn.Exec(ctx,
+		"INSERT INTO square_webhook_events (event_id, received_at) VALUES ($1, NOW() - INTERVAL '100 days')",
+		"evt_old"); err != nil {
+		t.Fatalf("failed to insert old webhook event: %v", err)
+	}
+	t.Cleanup(func() {
+		_, _ = db.Conn.Exec(ctx, "DELETE FROM square_webhook_events WHERE event_id IN ('evt_recent', 'evt_old')")
+	})
+
+	n, err := SweepSquareWebhookEvents(ctx)
+	if err != nil {
+		t.Fatalf("SweepSquareWebhookEvents failed: %v", err)
+	}
+	if n != 1 {
+		t.Errorf("expected 1 row deleted, got %d", n)
+	}
+
+	// Old row must be gone
+	var oldCount int
+	if err := db.Conn.QueryRow(ctx,
+		"SELECT COUNT(*) FROM square_webhook_events WHERE event_id = 'evt_old'").Scan(&oldCount); err != nil {
+		t.Fatalf("failed to count old event: %v", err)
+	}
+	if oldCount != 0 {
+		t.Errorf("expected old event to be deleted, got %d rows", oldCount)
+	}
+
+	// Recent row must remain
+	var recentCount int
+	if err := db.Conn.QueryRow(ctx,
+		"SELECT COUNT(*) FROM square_webhook_events WHERE event_id = 'evt_recent'").Scan(&recentCount); err != nil {
+		t.Fatalf("failed to count recent event: %v", err)
+	}
+	if recentCount != 1 {
+		t.Errorf("expected recent event to be preserved, got %d rows", recentCount)
+	}
+}
+
+func TestSweepSquareWebhookEvents_EmptyTable(t *testing.T) {
+	ctx := context.Background()
+
+	if _, err := db.Conn.Exec(ctx, "DELETE FROM square_webhook_events"); err != nil {
+		t.Fatalf("failed to clear square_webhook_events: %v", err)
+	}
+
+	n, err := SweepSquareWebhookEvents(ctx)
+	if err != nil {
+		t.Fatalf("SweepSquareWebhookEvents failed on empty table: %v", err)
+	}
+	if n != 0 {
+		t.Errorf("expected 0 rows deleted on empty table, got %d", n)
+	}
+}
diff --git a/backend/internal/jobs/scheduler_test.go b/backend/internal/jobs/scheduler_test.go
index ceeec01..b9356f9 100644
--- a/backend/internal/jobs/scheduler_test.go
+++ b/backend/internal/jobs/scheduler_test.go
@@ -413,8 +413,8 @@ func TestRegisterAll_RegistersExpectedJobs(t *testing.T) {
 	s := New()
 	RegisterAll(s)
 
-	if got := len(s.registry); got != 23 {
-		t.Fatalf("RegisterAll() registered %d jobs, want 23", got)
+	if got := len(s.registry); got != 24 {
+		t.Fatalf("RegisterAll() registered %d jobs, want 24", got)
 	}
 
 	registered := make(map[string]Job, len(s.registry))
@@ -490,6 +490,7 @@ func expectedJobNames() map[string]bool {
 		"transition-discount-campaigns":       true,
 		"cleanup-verification-codes":          true,
 		"cleanup-refresh-tokens":              true,
+		"sweep-square-webhook-events":         true,
 		"apply-default-hours":                 true,
 	}
 }
diff --git a/backend/testutils/testdb/testdb.go b/backend/testutils/testdb/testdb.go
index f8fda50..ac2ca45 100644
--- a/backend/testutils/testdb/testdb.go
+++ b/backend/testutils/testdb/testdb.go
@@ -53,6 +53,13 @@ func CreateTestDatabase(dbName string) *pgxpool.Pool {
 	}
 	defer adminPool.Close()
 
+	// Pre-flight check: pgxpool.New is lazy, so surface the admin connection
+	// failure here with actionable guidance instead of a confusing
+	// "database \"crussell_test_...\" does not exist" error further down.
+	if err := adminPool.Ping(ctx); err != nil {
+		log.Fatalf("testdb: run `docker compose up -d postgres` (see README Getting Started) — test database admin connection failed: %v", err)
+	}
+
 	safeName := pgx.Identifier{dbName}.Sanitize()
 
 	// Drop existing database with retries (terminate connections first)
diff --git a/init-scripts/init-script.sql b/init-scripts/init-script.sql
index f3276b9..fd94068 100644
--- a/init-scripts/init-script.sql
+++ b/init-scripts/init-script.sql
@@ -2284,6 +2284,18 @@ CREATE TABLE square_deposits (
 
 CREATE INDEX idx_square_deposits_deposited ON square_deposits(deposited_at);
 
+-- =======================================
+-- SQUARE WEBHOOK EVENTS TABLE (Webhook Dedup)
+-- =======================================
+-- Records every Square webhook event_id the handler has accepted. The webhook
+-- handler inserts with ON CONFLICT (event_id) DO NOTHING and treats a 0-row
+-- insert as a duplicate, giving restart-safe, at-most-once delivery with no
+-- eviction limit (unlike the in-memory fast-path cache).
+CREATE TABLE IF NOT EXISTS square_webhook_events (
+    event_id TEXT PRIMARY KEY,
+    received_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
 -- =======================================
 -- CARDDAV / CALDAV TABLES (used by SabreDAV sync in Go backend)
 -- =======================================