feat: Complete DI cleanup - migrate test helpers to Database struct

- Update internal/db/test_helpers.go to use Database struct instead of globals
- Update internal/server/test_helpers.go to use TestDatabase.Pool
- Add TODO comment to old Dbpool/Ctx globals in dbHelper.go
- Remove db.Testf() usage from production code (kept for deprecated /dbtest endpoint)

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

internal/server/test_helpers.go

@@ -51,19 +51,16 @@ func StartTestServer(t *testing.T) *echo.Echo {
 	// Initialize database for tests
 	db.TestSetupDB(t)
 	
-	// Initialize backend with the global Dbpool
+	// Initialize backend with test database pool
 	// This ensures BackendRepo() and BackendCtx() are available
-	if db.Dbpool != nil {
-		backend.InitBackend(db.Dbpool)
+	if db.TestDatabase != nil && db.TestDatabase.Pool != nil {
+		backend.InitBackend(db.TestDatabase.Pool)
 	}
 
 	// Create a Server instance and get its routes
 	s := &Server{
-		db: &db.Database{
-			Pool: db.Dbpool,
-			Ctx:  db.Ctx,
-		},
-		tokenHandler: NewTokenHandler(db.Dbpool),
+		db:            db.TestDatabase,
+		tokenHandler:   NewTokenHandler(db.TestDatabase.Pool),
 	}
 	handler := s.RegisterRoutes()