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/shape_ledger.py

@@ -31,6 +31,7 @@ from sqlalchemy import select
 
 from scribe.models import async_session
 from scribe.models.code_shape import CodeShape, CodeShapeEvent
+from scribe.models.base import iso
 
 logger = logging.getLogger(__name__)
 
@@ -1318,9 +1319,9 @@ async def shape_history(
                 "classified_by": r.classified_by, "reason": r.reason,
                 "first_seen_commit": r.first_seen_commit,
                 "last_seen_commit": r.last_seen_commit,
-                "first_seen_at": r.created_at.isoformat() if r.created_at else None,
-                "vanished_at": r.vanished_at.isoformat() if r.vanished_at else None,
-                "recheck_at": r.recheck_at.isoformat() if r.recheck_at else None,
+                "first_seen_at": iso(r.created_at),
+                "vanished_at": iso(r.vanished_at),
+                "recheck_at": iso(r.recheck_at),
                 "diverges_from": r.diverges_from,
             }
             for r in rows