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/db/db_test.go

@@ -15,7 +15,7 @@ func resetEnv() {
 	os.Setenv("POSTGRES_USER", "myuser")
 	os.Setenv("POSTGRES_PASSWORD", "mypassword")
 	os.Setenv("POSTGRES_HOST", "localhost")
-	os.Setenv("POSTGRES_DB", "crussell_test")
+	os.Setenv("POSTGRES_DB", "crussell_test_db")
 }
 
 func closePool() {
@@ -25,6 +25,8 @@ func closePool() {
 	}
 }
 
+var testDBName = "crussell_test_db"
+
 // =============================================================================
 // Happy path — connect + ping
 // =============================================================================
@@ -177,9 +179,4 @@ func TestGetEnv_ReturnsEmptyWhenUnset(t *testing.T) {
 // Clean-up — restore env after all tests
 // =============================================================================
 
-func TestMain(m *testing.M) {
-	resetEnv()
-	code := m.Run()
-	closePool()
-	os.Exit(code)
-}
+

backend/db/testmain_test.go

@@ -0,0 +1,21 @@
+//go:build test
+// +build test
+
+package db
+
+import (
+	"os"
+	"testing"
+
+	"crussell/testutils/testdb"
+)
+
+func TestMain(m *testing.M) {
+	pool := testdb.CreateTestDatabase(testDBName)
+	pool.Close() // db.Connect() creates its own pool
+	resetEnv()
+	code := m.Run()
+	closePool()
+	testdb.DestroyTestDatabase(nil, testDBName)
+	os.Exit(code)
+}