Add federation/ActivityPub models, contracts, and services

Phase 0+1 of ActivityPub integration:
- 6 ORM models (ActorProfile, APActivity, APFollower, APInboxItem, APAnchor, IPFSPin)
- FederationService protocol + SqlFederationService implementation + stub
- 4 DTOs (ActorProfileDTO, APActivityDTO, APFollowerDTO, APAnchorDTO)
- Registry slot for federation service
- Alembic migration for federation tables
- IPFS async client (httpx-based)
- HTTP Signatures (RSA-2048 sign/verify)
- login_url() now uses AUTH_APP env var for flexible auth routing

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

infrastructure/factory.py

@@ -11,7 +11,7 @@ from shared.config import init_config, config, pretty
 from shared.models import KV  # ensure shared models imported
 # Register all app model classes with SQLAlchemy so cross-domain
 # relationship() string references resolve correctly.
-for _mod in ("blog.models", "market.models", "cart.models", "events.models"):
+for _mod in ("blog.models", "market.models", "cart.models", "events.models", "federation.models"):
     try:
         __import__(_mod)
     except ImportError: