Add federation event handlers, AP delivery, and anchoring

Phase 3-5 of ActivityPub integration:
- Federation handlers: post.published, calendar_entry.created, product.listed
  → publish_activity() for AP outbox
- AP delivery handler: federation.activity_created → sign + POST to follower
  inboxes with HTTP Signatures
- IPFS storage wired into publish_activity() (best-effort)
- Anchoring utility: merkle trees + OpenTimestamps Bitcoin timestamping

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

services/federation_impl.py

@@ -158,7 +158,28 @@ class SqlFederationService:
         session.add(activity)
         await session.flush()
 
-        # Emit domain event for downstream processing (IPFS storage, delivery)
+        # Store activity JSON on IPFS (best-effort — don't fail publish if IPFS down)
+        try:
+            from shared.utils.ipfs_client import add_json, is_available
+            if await is_available():
+                activity_json = {
+                    "@context": "https://www.w3.org/ns/activitystreams",
+                    "id": activity_uri,
+                    "type": activity_type,
+                    "actor": f"https://{domain}/users/{username}",
+                    "published": now.isoformat(),
+                    "object": {
+                        "type": object_type,
+                        **object_data,
+                    },
+                }
+                cid = await add_json(activity_json)
+                activity.ipfs_cid = cid
+                await session.flush()
+        except Exception:
+            pass  # IPFS failure is non-fatal
+
+        # Emit domain event for downstream processing (delivery)
         from shared.events import emit_event
         await emit_event(
             session,