refactor(backend): migrate test infrastructure to isolated databases

Add CreateTestDatabase function for parallel isolated test databases per package.

- Add CreateTestDatabase() for isolated test DBs (parallel-safe)
- Move all TestMain functions to per-package testmain_test.go files
- Remove old TestMain from handlers_test.go, jwt_test.go, main_test.go
- Add JWT init guard in main.go to skip when -test.* flags detected
- Update testdb.go with admin DSN and proper cleanup
- Rename test database to crussell_test_db for consistency
- Replace testdb.NewPool + testdb.Migrate pattern with CreateTestDatabase

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/handlers/user/testmain_test.go

@@ -13,14 +13,10 @@ import (
 )
 
 func TestMain(m *testing.M) {
-	pool, err := testdb.NewPool("")
-	if err != nil {
-		panic(err)
-	}
-	testdb.Migrate(&testing.T{}, pool)
+	pool := testdb.CreateTestDatabase("crussell_test_handlers_user")
 	db.DB = pool
 	jwt.Init()
 	code := m.Run()
-	pool.Close()
+	testdb.DestroyTestDatabase(pool, "crussell_test_handlers_user")
 	os.Exit(code)
 }