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>

services/registry.py

@@ -21,6 +21,7 @@ from shared.contracts.protocols import (
     CalendarService,
     MarketService,
     CartService,
+    FederationService,
 )
 
 
@@ -37,6 +38,7 @@ class _ServiceRegistry:
         self._calendar: CalendarService | None = None
         self._market: MarketService | None = None
         self._cart: CartService | None = None
+        self._federation: FederationService | None = None
 
     # -- blog -----------------------------------------------------------------
     @property
@@ -82,6 +84,17 @@ class _ServiceRegistry:
     def cart(self, impl: CartService) -> None:
         self._cart = impl
 
+    # -- federation -----------------------------------------------------------
+    @property
+    def federation(self) -> FederationService:
+        if self._federation is None:
+            raise RuntimeError("FederationService not registered")
+        return self._federation
+
+    @federation.setter
+    def federation(self, impl: FederationService) -> None:
+        self._federation = impl
+
     # -- introspection --------------------------------------------------------
     def has(self, name: str) -> bool:
         """Check whether a domain service is registered."""