Add IPFS node to L2 for federated content storage

- Add IPFS container to docker-compose
- Add ipfshttpclient dependency
- Add ipfs_client.py module for IPFS operations
- Add ipfs_cid field to Asset model and database schema
- Pin content on L2 IPFS when assets are published/registered

L2 now stores content on its own IPFS node, enabling
federation - content remains available even if L1 goes down.

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

docker-compose.yml

@@ -17,6 +17,17 @@ services:
       timeout: 5s
       retries: 5
 
+  ipfs:
+    image: ipfs/kubo:latest
+    volumes:
+      - ipfs_data:/data/ipfs
+    networks:
+      - internal
+    deploy:
+      replicas: 1
+      restart_policy:
+        condition: on-failure
+
   l2-server:
     image: git.rose-ash.com/art-dag/l2-server:latest
     env_file:
@@ -24,6 +35,7 @@ services:
     environment:
       - ARTDAG_DATA=/data/l2
       - DATABASE_URL=postgresql://artdag:${POSTGRES_PASSWORD:-artdag}@postgres:5432/artdag
+      - IPFS_API=/dns/ipfs/tcp/5001
       # ARTDAG_DOMAIN, ARTDAG_USER, JWT_SECRET from .env file
     volumes:
       - l2_data:/data/l2  # Still needed for RSA keys
@@ -32,6 +44,7 @@ services:
       - externalnet
     depends_on:
       - postgres
+      - ipfs
     deploy:
       replicas: 1
       restart_policy:
@@ -40,6 +53,7 @@ services:
 volumes:
   l2_data:
   postgres_data:
+  ipfs_data:
 
 networks:
   internal: