Add webhook-event retention job, schema, and testdb pre-flight hint

square_webhook_events is now CREATE TABLE IF NOT EXISTS, registered as the 24th maintenance job (sweep-square-webhook-events, daily 2:30am) pruning rows older than 90 days, and testdb gives a clear docker compose hint when the admin DB connection fails.

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)