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/__init__.py

@@ -0,0 +1,16 @@
+"""
+Database package for Core-API
+
+Provides async PostgreSQL database connectivity using SQLAlchemy 2.0.
+"""
+from src.db.database import (
+    get_async_session,
+    get_database,
+    Database,
+)
+
+__all__ = [
+    "get_async_session",
+    "get_database",
+    "Database",
+]