Persistent User & Session Storage: Replace In-Memory with Database

[W1ndrunn3rr]

Replace in-memory SessionManager with a real database (PostgreSQL or MongoDB).

• Add DB as a service in compose.stack.yml
• Migrate ConversationSession and Message models to SQLAlchemy/Motor (async)
• Persist: user_id, session_id, messages[], created_at, is_active, metadata
• Implement user registration/auth (JWT or API key; reject anonymous user)
• Refactor session_manager.py for async DB calls, remove threading.Lock