Add NOTIFY/LISTEN wake-up to event processor

emit_activity() now fires NOTIFY ap_activity_pending inside the
caller's transaction (delivered on commit).  EventProcessor maintains
a dedicated asyncpg LISTEN connection and wakes the poll loop
immediately, dropping latency from ~2 s to sub-100 ms.  The fixed-
interval poll remains as a safety-net fallback.

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

events/bus.py

@@ -14,6 +14,7 @@ import uuid
 from collections import defaultdict
 from typing import Awaitable, Callable, Dict, List, Tuple
 
+from sqlalchemy import text
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from shared.models.federation import APActivity
@@ -119,4 +120,7 @@ async def emit_activity(
     )
     session.add(activity)
     await session.flush()
+    # Wake any listening EventProcessor as soon as this transaction commits.
+    # NOTIFY is transactional — delivered only after commit.
+    await session.execute(text("NOTIFY ap_activity_pending"))
     return activity