Add debug logging to EventProcessor and activity handler registry

Logs which handlers are registered at startup and which handlers are
found/called when processing each activity.

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

events/bus.py

@@ -9,6 +9,7 @@ EventProcessor dispatches when processing pending activities.
 """
 from __future__ import annotations
 
+import logging
 import uuid
 from collections import defaultdict
 from typing import Awaitable, Callable, Dict, List, Tuple
@@ -17,6 +18,8 @@ from sqlalchemy.ext.asyncio import AsyncSession
 
 from shared.models.federation import APActivity
 
+log = logging.getLogger(__name__)
+
 # ---------------------------------------------------------------------------
 # Activity-handler registry
 # ---------------------------------------------------------------------------
@@ -40,6 +43,7 @@ def register_activity_handler(
     """
     key = (activity_type, object_type or "*")
     _activity_handlers[key].append(fn)
+    log.info("Registered activity handler %s.%s for key %s", fn.__module__, fn.__qualname__, key)
 
 
 def get_activity_handlers(