Add IPFS_PRIMARY environment variable for server

When IPFS_PRIMARY=true:
- /api/run-recipe uses run_recipe_cid task
- Recipe registered on IPFS before execution
- Input CIDs fetched from cache manager
- Everything flows through IPFS, no local cache

Usage in docker-compose:
  environment:
    - IPFS_PRIMARY=true

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

docker-compose.yml

@@ -52,6 +52,8 @@ services:
       - DATABASE_URL=postgresql://artdag:artdag@postgres:5432/artdag
       - IPFS_API=/dns/ipfs/tcp/5001
       - CACHE_DIR=/data/cache
+      # Set IPFS_PRIMARY=true to use IPFS-primary mode (everything on IPFS, no local cache)
+      # - IPFS_PRIMARY=true
       # L2_SERVER, L2_DOMAIN, IPFS_GATEWAY_URL from .env file
     volumes:
       - l1_cache:/data/cache