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>

alembic/env.py

@@ -19,7 +19,7 @@ from shared.db.base import Base
 
 # Import ALL models so Base.metadata sees every table
 import shared.models  # noqa: F401  User, KV, MagicLink, MenuItem, Ghost*
-for _mod in ("blog.models", "market.models", "cart.models", "events.models", "glue.models"):
+for _mod in ("blog.models", "market.models", "cart.models", "events.models", "federation.models", "glue.models"):
     try:
         __import__(_mod)
     except ImportError: