Migrate to PostgreSQL database, consolidate routes, improve home page

- Add PostgreSQL with asyncpg for persistent storage
- Create db.py module with async database operations
- Create migrate.py script to migrate JSON data to PostgreSQL
- Update docker-compose.yml with PostgreSQL service
- Home page now shows README with styled headings
- Remove /ui prefix routes, use content negotiation on main routes
- Add /activities/{idx} as canonical route (with /activity redirect)
- Update /assets/{name} to support HTML and JSON responses
- Convert auth.py to use async database operations
- RSA keys still stored as files in $ARTDAG_DATA/keys/

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

requirements.txt

@@ -6,3 +6,4 @@ bcrypt>=4.0.0
 python-jose[cryptography]>=3.3.0
 markdown>=3.5.0
 python-multipart>=0.0.6
+asyncpg>=0.29.0