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/warranty/api/routes.py

@@ -77,6 +77,11 @@ def list_warranties():
     query = Warranty.query
     if request.args.get('active', 'true').lower() != 'false':
         query = query.filter_by(isactive=True)
+    # Exact-match natural-key lookup for idempotent import (servicetag + vendor).
+    if exactservicetag := request.args.get('servicetag'):
+        query = query.filter(Warranty.servicetag == exactservicetag)
+    if exactvendor := request.args.get('vendor'):
+        query = query.filter(Warranty.vendor == exactvendor)
     assetid = request.args.get('assetid', type=int)
     if assetid:
         query = (query.join(WarrantyAsset, WarrantyAsset.warrantyid == Warranty.warrantyid)