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>

alembic/README

@@ -0,0 +1,21 @@
+Alembic Migrations for Core-API
+
+This directory contains database migrations managed by Alembic.
+
+Commands:
+  # Generate a new migration (after changing models)
+  alembic revision --autogenerate -m "description"
+
+  # Apply all pending migrations
+  alembic upgrade head
+
+  # Rollback last migration
+  alembic downgrade -1
+
+  # View migration history
+  alembic history
+
+  # View current revision
+  alembic current
+
+See https://alembic.sqlalchemy.org for more documentation.