feat(versions): daily 03:00 UTC auto-pin scan scheduler

BackgroundScheduler with a single CronTrigger fires scan_all_users_for
_auto_pins via asyncio.run_coroutine_threadsafe (mirrors the journal-
scheduler pattern). Wired into app startup/shutdown alongside the other
schedulers.

src/fabledassistant/services/version_pinning_scheduler.py

@@ -0,0 +1,72 @@
+"""Daily APScheduler cron for the auto-pin scan.
+
+Single global job at 03:00 UTC. Runs scan_all_users_for_auto_pins so the
+system promotes stable note versions before they get aged out of the
+rolling cap. Off-hours by design — the scan is cheap but not time-
+critical and doesn't need to interrupt regular activity.
+
+Mirrors the BackgroundScheduler + threadsafe-async-call pattern used by
+journal_scheduler.py.
+"""
+from __future__ import annotations
+
+import asyncio
+import logging
+
+from apscheduler.schedulers.background import BackgroundScheduler
+from apscheduler.triggers.cron import CronTrigger
+
+from fabledassistant.services.version_pinning import scan_all_users_for_auto_pins
+
+logger = logging.getLogger(__name__)
+
+_scheduler: BackgroundScheduler | None = None
+_loop: asyncio.AbstractEventLoop | None = None
+
+
+def _run_scan_threadsafe() -> None:
+    """APScheduler invokes this from a worker thread; bridge into the
+    asyncio loop so the scan can await its DB operations."""
+    if _loop is None:
+        logger.warning("version_pinning scheduler: no loop registered")
+        return
+
+    async def _runner():
+        try:
+            results = await scan_all_users_for_auto_pins()
+            total = sum(results.values())
+            if total > 0:
+                logger.info(
+                    "auto-pin scan: pinned %d version(s) across %d user(s)",
+                    total, len(results),
+                )
+            else:
+                logger.debug("auto-pin scan: no new pins")
+        except Exception:
+            logger.exception("auto-pin scan run failed")
+
+    asyncio.run_coroutine_threadsafe(_runner(), _loop)
+
+
+def start_version_pinning_scheduler(loop: asyncio.AbstractEventLoop) -> None:
+    global _scheduler, _loop
+    if _scheduler is not None:
+        return
+    _loop = loop
+    _scheduler = BackgroundScheduler()
+    _scheduler.add_job(
+        _run_scan_threadsafe,
+        trigger=CronTrigger(hour=3, minute=0, timezone="UTC"),
+        id="version_pinning_auto_scan",
+        replace_existing=True,
+    )
+    _scheduler.start()
+    logger.info("Version pinning scheduler started (daily 03:00 UTC)")
+
+
+def stop_version_pinning_scheduler() -> None:
+    global _scheduler
+    if _scheduler is not None:
+        _scheduler.shutdown(wait=False)
+        _scheduler = None
+        logger.info("Version pinning scheduler stopped")