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/handlers/admin/testmain_test.go

@@ -0,0 +1,26 @@
+//go:build test
+// +build test
+
+package admin
+
+import (
+	"os"
+	"testing"
+
+	"crussell/db"
+	"crussell/testutils/testdb"
+	"crussell/testutils/jwt"
+)
+
+func TestMain(m *testing.M) {
+	pool, err := testdb.NewPool("")
+	if err != nil {
+		panic(err)
+	}
+	testdb.Migrate(&testing.T{}, pool)
+	db.DB = pool
+	jwt.Init()
+	code := m.Run()
+	pool.Close()
+	os.Exit(code)
+}