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

alembic/script.py.mako

@@ -0,0 +1,26 @@
+"""${message}
+
+Revision ID: ${up_revision}
+Revises: ${down_revision | comma,n}
+Create Date: ${create_date}
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+${imports if imports else ""}
+
+# revision identifiers, used by Alembic.
+revision: str = ${repr(up_revision)}
+down_revision: Union[str, None] = ${repr(down_revision)}
+branch_labels: Union[str, Sequence[str], None] = ${repr(branch_labels)}
+depends_on: Union[str, Sequence[str], None] = ${repr(depends_on)}
+
+
+def upgrade() -> None:
+    ${upgrades if upgrades else "pass"}
+
+
+def downgrade() -> None:
+    ${downgrades if downgrades else "pass"}