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/guest.go

@@ -92,7 +92,7 @@ func CreateGuestUserHandler(w http.ResponseWriter, r *http.Request) {
 
 	// Check if email already exists with a registered (non-guest) role
 	var existingRole string
-	err = db.DB.QueryRow(r.Context(), `
+	err = db.Conn.QueryRow(r.Context(), `
 		SELECT account_role FROM users WHERE email = $1
 	`, req.Email).Scan(&existingRole)
 
@@ -106,7 +106,7 @@ func CreateGuestUserHandler(w http.ResponseWriter, r *http.Request) {
 
 	// Create new guest user
 	var userID string
-	err = db.DB.QueryRow(r.Context(), `
+	err = db.Conn.QueryRow(r.Context(), `
 		INSERT INTO users
 			(n_first_name, n_last_name, email, phone, date_of_birth,
 			 account_role, account_type, password_hash, privacy_policy_and_terms_consent)
@@ -148,7 +148,7 @@ func CheckEmailHandler(w http.ResponseWriter, r *http.Request) {
 	phone := strings.TrimSpace(r.URL.Query().Get("phone"))
 
 	var dbFirstName, dbLastName, dbPhone *string
-	err := db.DB.QueryRow(r.Context(), `
+	err := db.Conn.QueryRow(r.Context(), `
 		SELECT n_first_name, n_last_name, phone
 		FROM users
 		WHERE email = $1 AND account_role != 'guest'