feat: add authentication and user management with Authentik integration

- Add PostgreSQL database with async SQLAlchemy
- Add Alembic migrations for schema management
- Add User, Role, UserPreferences, ApiKey models
- Add auth endpoints: /auth/me, /auth/users, /auth/users/sync-from-authentik
- Add token validation via Authentik userinfo endpoint
- Add bulk user sync from Authentik admin API
- Add database health check to diagnostics

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

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

src/db/models/__init__.py

@@ -0,0 +1,17 @@
+"""
+SQLAlchemy Models for Core-API
+
+Database models for authentication, authorization, and user management.
+"""
+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
+
+__all__ = [
+    "User",
+    "Role",
+    "UserRole",
+    "UserPreferences",
+    "ApiKey",
+]