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/testmain_test.go

@@ -0,0 +1,20 @@
+//go:build test
+// +build test
+
+package main
+
+import (
+	"os"
+	"testing"
+
+	"crussell/db"
+	"crussell/testutils/testdb"
+)
+
+func TestMain(m *testing.M) {
+	pool := testdb.CreateTestDatabase("crussell_test")
+	db.DB = pool
+	code := m.Run()
+	testdb.DestroyTestDatabase(pool, "crussell_test")
+	os.Exit(code)
+}