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/user/loyalty.go

@@ -18,7 +18,7 @@ func GetLoyaltyHandler(w http.ResponseWriter, r *http.Request) {
 	userID, _ := mw.GetUserID(r.Context())
 
 	var loyalty LoyaltyResponse
-	err := db.DB.QueryRow(r.Context(), `
+	err := db.Conn.QueryRow(r.Context(), `
 		SELECT loyalty_stamps, referral_code
 		FROM users
 		WHERE id = $1