refactor(services): one periodic-task shape, one APScheduler job shape, one token hash, one summary rule — the services pass of the shape audit (#2830, milestone 296)

- background.start_periodic(interval, work, label=) replaces the three hand-rolled
  while-True/sleep/try loops in logging, auth and notifications.
- services/scheduler.ScheduledJob replaces the four private BackgroundScheduler
  copies in recurrence/version_pinning/trash/db_maintenance schedulers; public
  start_/stop_/reschedule_ surfaces unchanged.
- api_keys.hash_token is the one sha256 helper; auth.py used to inline it 5x.
- auth.is_registration_open reads via settings.get_admin_setting; notification
  prefs read via settings.get_setting; _fire_share_email uses _get_user_email.
- projects.get_project_summary / milestones.get_project_milestone_summary are
  now the one-id view of their batch siblings instead of a second copy of the
  queries; sharing.best_permission_by (was _deduplicate_by_permission) is the
  one rank-dedup, now also used by list_projects_for_user.
- backup: the row builders for every section both exporters carry are named
  functions, so a column added to one export cannot silently miss the other.
- iso() from models.base replaces the attr.isoformat()-if-attr-else-None idiom
  and db_maintenance._iso across services; backup keeps its explicit shape.
- trash.py hoists the sql_delete/timedelta imports it re-imported per function.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/scribe/services/background.py

@@ -45,6 +45,26 @@ def spawn(coro: Coroutine, *, site: str) -> None:
     task.add_done_callback(_done)
 
 
+def start_periodic(interval_s: float, work, *, label: str) -> asyncio.Task:
+    """A forever loop that sleeps ``interval_s`` then awaits ``work()``, logging
+    (never raising) when a tick fails — the one shape the hourly/daily
+    retention sweeps share (log retention, notification sweep, auth-token
+    purge). Sleeps FIRST so startup isn't a sweep; holds a strong reference
+    like spawn() so the loop cannot be garbage-collected mid-flight."""
+    async def _loop() -> None:
+        while True:
+            await asyncio.sleep(interval_s)
+            try:
+                await work()
+            except Exception:
+                logger.exception("periodic task %s failed", label)
+
+    task = asyncio.get_running_loop().create_task(_loop(), name=f"periodic-{label}")
+    _pending.add(task)
+    task.add_done_callback(_pending.discard)
+    return task
+
+
 async def drain() -> None:
     """Await everything in flight — for tests that need the writes landed."""
     while _pending: