Add PostgreSQL memory system for persistent user and conversation storage

- Add PostgreSQL database with SQLAlchemy async support
- Create models: User, UserFact, UserPreference, Conversation, Message, Guild, GuildMember
- Add custom name support so bot knows 'who is who'
- Add user facts system for remembering things about users
- Add persistent conversation history that survives restarts
- Add memory commands cog (!setname, !remember, !whatdoyouknow, !forgetme)
- Add admin commands (!setusername, !teachbot)
- Set up Alembic for database migrations
- Update docker-compose with PostgreSQL service
- Gracefully falls back to in-memory storage when DB not configured

requirements.txt

@@ -13,3 +13,8 @@ aiohttp>=3.9.0
 pydantic>=2.6.0
 pydantic-settings>=2.2.0
 python-dotenv>=1.0.0
+
+# Database
+asyncpg>=0.29.0
+sqlalchemy[asyncio]>=2.0.0
+alembic>=1.13.0