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>

docker-compose.yml

@@ -1,17 +1,38 @@
 version: "3.8"
 
 services:
+  postgres:
+    image: postgres:16-alpine
+    environment:
+      POSTGRES_USER: artdag
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:-artdag}
+      POSTGRES_DB: artdag
+    volumes:
+      - postgres_data:/var/lib/postgresql/data
+    networks:
+      - internal
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U artdag"]
+      interval: 5s
+      timeout: 5s
+      retries: 5
+
   l2-server:
     image: git.rose-ash.com/art-dag/l2-server:latest
     env_file:
       - .env
     environment:
       - ARTDAG_DATA=/data/l2
+      - DATABASE_URL=postgresql://artdag:${POSTGRES_PASSWORD:-artdag}@postgres:5432/artdag
       # ARTDAG_DOMAIN, ARTDAG_USER, ARTDAG_L1, JWT_SECRET from .env file
     volumes:
-      - l2_data:/data/l2
+      - l2_data:/data/l2  # Still needed for RSA keys
     networks:
+      - internal
       - externalnet
+    depends_on:
+      postgres:
+        condition: service_healthy
     deploy:
       replicas: 1
       restart_policy:
@@ -19,7 +40,9 @@ services:
 
 volumes:
   l2_data:
+  postgres_data:
 
 networks:
+  internal:
   externalnet:
     external: true