sx-pub Phase 3: federation — outbox, inbox, follow, delivery, HTTP signatures

New endpoints:
- GET /pub/outbox — paginated activity feed
- POST /pub/inbox — receive Follow/Accept/Publish from remote servers
- POST /pub/follow — follow a remote sx-pub server
- GET /pub/followers — list accepted followers
- GET /pub/following — list who we follow

Federation mechanics:
- HTTP Signature generation (RSA-SHA256) for signed outgoing requests
- HTTP Signature verification for incoming requests
- Auto-accept Follow → store follower → send Accept back
- Accept handling → update following state
- Publish mirroring → pin remote CID to local IPFS
- deliver_to_followers → fan out signed activities to all follower inboxes
- Publish now records activity in outbox for federation delivery

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sx/sxc/pages/helpers.py

@@ -45,6 +45,13 @@ def _register_sx_helpers() -> None:
         "pub-collection-items": _pub_collection_items,
         "pub-resolve-document": _pub_resolve_document,
         "pub-fetch-cid": _pub_fetch_cid,
+        "pub-outbox-data": _pub_outbox_data,
+        "pub-followers-data": _pub_followers_data,
+        "pub-following-data": _pub_following_data,
+        "pub-follow-remote": _pub_follow_remote,
+        "pub-process-inbox": _pub_process_inbox,
+        "pub-deliver-to-followers": _pub_deliver_to_followers,
+        "pub-request-body": _pub_request_body,
     })
 
 
@@ -1764,3 +1771,38 @@ async def _pub_resolve_document(collection_slug, slug):
 async def _pub_fetch_cid(cid):
     from .pub_helpers import fetch_cid
     return await fetch_cid(cid)
+
+
+async def _pub_outbox_data(page=""):
+    from .pub_helpers import outbox_data
+    return await outbox_data(page)
+
+
+async def _pub_followers_data():
+    from .pub_helpers import followers_data
+    return await followers_data()
+
+
+async def _pub_following_data():
+    from .pub_helpers import following_data
+    return await following_data()
+
+
+async def _pub_follow_remote(actor_url):
+    from .pub_helpers import follow_remote
+    return await follow_remote(actor_url)
+
+
+async def _pub_process_inbox(body_sx):
+    from .pub_helpers import process_inbox
+    return await process_inbox(body_sx)
+
+
+async def _pub_deliver_to_followers(activity_sx):
+    from .pub_helpers import deliver_to_followers
+    return await deliver_to_followers(activity_sx)
+
+
+async def _pub_request_body():
+    from .pub_helpers import get_request_body
+    return await get_request_body()