Fix AP Accept: add activity id + upsert followers

- Accept activity now includes required 'id' field (uuid-based)
- Updated shared submodule: add_follower upserts to prevent IntegrityError

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

bp/actors/routes.py

@@ -7,6 +7,7 @@ from __future__ import annotations
 import json
 import logging
 import os
+import uuid
 
 import httpx
 from quart import Blueprint, request, abort, Response, g, render_template
@@ -52,8 +53,10 @@ async def _send_accept(
     username = actor.preferred_username
     actor_url = f"https://{domain}/users/{username}"
 
+    accept_id = f"{actor_url}/activities/{uuid.uuid4()}"
     accept = {
         "@context": "https://www.w3.org/ns/activitystreams",
+        "id": accept_id,
         "type": "Accept",
         "actor": actor_url,
         "object": follow_activity,