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>

requirements.txt

@@ -7,3 +7,4 @@ python-jose[cryptography]>=3.3.0
 markdown>=3.5.0
 python-multipart>=0.0.6
 asyncpg>=0.29.0
+ipfshttpclient>=0.7.0