feat: add groups management API

- GET /auth/groups - list groups with search/pagination
- POST /auth/groups/sync-from-authentik - bulk sync from Authentik
- Group model and database migration

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/db/models/__init__.py

@@ -7,6 +7,7 @@ from src.db.models.user import User
 from src.db.models.role import Role, UserRole
 from src.db.models.user_preferences import UserPreferences
 from src.db.models.api_key import ApiKey
+from src.db.models.group import Group
 
 __all__ = [
     "User",
@@ -14,4 +15,5 @@ __all__ = [
     "UserRole",
     "UserPreferences",
     "ApiKey",
+    "Group",
 ]