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/logging.py

@@ -194,18 +194,14 @@ async def delete_old_logs(retention_days: int) -> int:
         return result.rowcount
 
 
-async def _retention_loop() -> None:
-    while True:
-        await asyncio.sleep(3600)  # hourly
-        try:
-            deleted = await delete_old_logs(Config.LOG_RETENTION_DAYS)
-            if deleted:
-                logger.info("Log retention: deleted %d old log entries", deleted)
-        except Exception:
-            logger.exception("Error in log retention cleanup")
+async def _retention_tick() -> None:
+    deleted = await delete_old_logs(Config.LOG_RETENTION_DAYS)
+    if deleted:
+        logger.info("Log retention: deleted %d old log entries", deleted)
 
 
 def start_log_retention_loop() -> None:
     global _retention_task
     if _retention_task is None or _retention_task.done():
-        _retention_task = asyncio.create_task(_retention_loop())
+        from scribe.services.background import start_periodic
+        _retention_task = start_periodic(3600, _retention_tick, label="log_retention")  # hourly