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/testdb/testdb.go

@@ -204,6 +204,7 @@ func TruncateTables(t *testing.T, pool *pgxpool.Pool) {
         "booking_services",
         "payments",
         "bookings",
+        "booking_edit_requests",
         "user_patch_tests",
         "patch_tests",
         "services",
@@ -212,8 +213,10 @@ func TruncateTables(t *testing.T, pool *pgxpool.Pool) {
         "user_notification_preferences",
         "time_blockers",
         "working_hours",
+        "exceptional_group_applications",
         "exceptional_working_hours",
         "exceptional_working_hours_groups",
+        "business_settings",
         "users",
         "images",
         "tags",