refactor: optimize test DB setup — TestMain per package, truncate-only between tests

- Add TestMain to all 10 test packages (schema DROP+CREATE runs once per package)
- Convert per-test setupTestDB to resetTestData (TRUNCATE only, ~60% faster)
- Add 3 missing tables to TruncateTables (booking_edit_requests, exceptional_group_applications, business_settings)
- Remove dead truncateDiscountTables helper
- Consolidate discount_test.go into package bookings (was external test package)
- Update testutils.SetupTestDB to truncate-only
- Fix unused imports across user, bookings, and handlers packages
- Verify: 286 passing, 2 skipped, 0 failures with -count=2 (no state leakage)

backend/testutils/helpers.go

@@ -19,25 +19,14 @@ import (
 	"github.com/jackc/pgx/v5/pgxpool"
 )
 
-// SetupTestDB initializes a test database and returns a cleanup function
-// Replaces the global db.DB with a test pool
+// SetupTestDB resets test data by truncating tables
+// Assumes db.DB is already set by TestMain
 func SetupTestDB(t *testing.T) func() {
 	t.Helper()
 
-	pool := testdb.Pool(t)
-	testdb.Migrate(t, pool)
+	testdb.TruncateTables(t, db.DB)
 
-	// Replace global db.DB with test pool
-	originalDB := db.DB
-	db.DB = pool
-
-	// Initialize JWT for tests
-	jwt.Init()
-
-	return func() {
-		db.DB = originalDB
-		pool.Close()
-	}
+	return func() {}
 }
 
 // MakeRequest makes an HTTP request to a handler with optional JWT token