fix(briefing): deadlock in scheduler startup

start_briefing_scheduler was called from before_serving (event loop thread)
and used run_coroutine_threadsafe(...).result() which blocks the calling
thread waiting for the coroutine to complete — but since the calling thread
IS the event loop, the coroutine could never run, causing a 10s timeout and
zero jobs scheduled.

Fix: make start_briefing_scheduler async and await _get_briefing_enabled_users()
directly. Also use asyncio.create_task for the catch-up rather than
run_coroutine_threadsafe. The background thread jobs (_run_user_slot_sync)
continue to use run_coroutine_threadsafe correctly since they run on the
APScheduler thread, not the event loop thread.

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

src/fabledassistant/app.py

@@ -246,7 +246,7 @@ def create_app() -> Quart:
 
         # Start briefing scheduler
         from fabledassistant.services.briefing_scheduler import start_briefing_scheduler
-        start_briefing_scheduler(asyncio.get_running_loop())
+        await start_briefing_scheduler(asyncio.get_running_loop())
 
     @app.after_serving
     async def shutdown():

src/fabledassistant/services/briefing_scheduler.py

@@ -296,10 +296,10 @@ async def _catchup_missed_slots(loop: asyncio.AbstractEventLoop) -> None:
                 )
 
 
-def start_briefing_scheduler(loop: asyncio.AbstractEventLoop) -> None:
+async def start_briefing_scheduler(loop: asyncio.AbstractEventLoop) -> None:
     """
     Start the APScheduler background scheduler with per-user timezone-aware jobs.
-    Must be called from the app's before_serving hook with the running event loop.
+    Must be awaited from the app's before_serving hook (async context).
     """
     global _scheduler, _loop
     if _scheduler is not None:
@@ -308,10 +308,10 @@ def start_briefing_scheduler(loop: asyncio.AbstractEventLoop) -> None:
     _loop = loop
     _scheduler = BackgroundScheduler(timezone="UTC")
 
-    # Schedule jobs synchronously: run the async query in the provided loop
+    # Await directly — we're already on the event loop, so run_coroutine_threadsafe
-    future = asyncio.run_coroutine_threadsafe(_get_briefing_enabled_users(), loop)
+    # would deadlock (it blocks the calling thread, which IS the event loop thread).
     try:
-        users = future.result(timeout=10)
+        users = await _get_briefing_enabled_users()
     except Exception:
         logger.exception("Failed to load briefing users at startup")
         users = []
@@ -325,7 +325,7 @@ def start_briefing_scheduler(loop: asyncio.AbstractEventLoop) -> None:
         len(users), len(users) * len(SLOTS),
     )
 
-    asyncio.run_coroutine_threadsafe(_catchup_missed_slots(loop), loop)
+    asyncio.create_task(_catchup_missed_slots(loop))
 
 
 def stop_briefing_scheduler() -> None: