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/auth/jwt_test.go

@@ -14,35 +14,13 @@ package auth
 
 import (
 	"context"
-	"fmt"
-	"os"
 	"strings"
 	"testing"
 	"time"
 
 	"crussell/db"
-	"crussell/testutils/testdb"
 )
 
-func TestMain(m *testing.M) {
-	InitJWT("test-secret-key-for-jwt-test")
-
-	pool, err := testdb.NewPool("")
-	if err != nil {
-		fmt.Fprintf(os.Stderr, "WARN: No test DB available — JTI revocation tests will fail: %v\n", err)
-	} else {
-		// Run migration to ensure the revoked_jtis and refresh_tokens tables exist
-		testdb.Migrate(&testing.T{}, pool)
-		db.DB = pool
-	}
-
-	code := m.Run()
-	if pool != nil {
-		pool.Close()
-	}
-	os.Exit(code)
-}
-
 // requiresDB skips the test if the database is not available (e.g. running
 // tests standalone without test DB setup). DB-backed JTI revocation tests
 // need a connection to the revoked_jtis table.

backend/auth/testmain_test.go

@@ -0,0 +1,23 @@
+//go:build test
+// +build test
+
+package auth
+
+import (
+	"os"
+	"testing"
+
+	"crussell/db"
+	"crussell/testutils/testdb"
+)
+
+func TestMain(m *testing.M) {
+	InitJWT("test-secret-key-for-jwt-test")
+
+	pool := testdb.CreateTestDatabase("crussell_test_auth")
+	db.DB = pool
+
+	code := m.Run()
+	testdb.DestroyTestDatabase(pool, "crussell_test_auth")
+	os.Exit(code)
+}