Add the API import surface for legacy migrations (contract 0.8.0)

Goal: an LLM or script can migrate an entire legacy database using only
the HTTP API - original history preserved, safely re-runnable.

- X-Import-Mode header (admin only): create/update endpoints across 15
  timestamped entity types accept original createddate/modifieddate;
  helper exposed via shopdb.api (contract 0.7.0 -> 0.8.0).
- Exact-match natural-key lookup filters on 13 list endpoints for the
  lookup-then-upsert recipe.
- Selfhosted USB checkout/checkin accept backdated event times in
  import mode.
- docs/IMPORT-API.md: operator manual grounded in the real legacy
  schema - order of operations, full table-by-table mapping including
  the machines fan-out, idempotent Python importer with dry-run, parity
  checks, and decided dispositions for unmigrated tables (DNC config
  stays live-fed via the collector; supportteams/appowners map to the
  upcoming supportteams model).

635 tests pass; naming green; frontend untouched.

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

plugins/notifications/api/routes.py

@@ -288,6 +288,11 @@ def list_notifications():
     if type_id := request.args.get('typeid', request.args.get('type_id')):
         query = query.filter(Notification.notificationtypeid == int(type_id))
 
+    # Exact-match lookup for idempotent import. Notifications have no strong
+    # natural key; ticketnumber is the best available when a ticket is set.
+    if exactticket := request.args.get('ticketnumber'):
+        query = query.filter(Notification.ticketnumber == exactticket)
+
     # Current filter (active based on dates)
     if request.args.get('current', 'false').lower() == 'true':
         now = datetime.now(timezone.utc).replace(tzinfo=None)