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>

shopdb/core/api/models.py

@@ -14,6 +14,7 @@ from shopdb.utils.responses import (
 from shopdb.utils.pagination import get_pagination_params, paginate_query
 
 from shopdb.utils.authz import require_permission, require_role
+from shopdb.utils.import_mode import apply_import_timestamps
 
 models_bp = Blueprint('models', __name__)
 
@@ -35,6 +36,11 @@ def list_models():
     if modeltype_id := request.args.get('modeltype', type=int):
         query = query.filter(Model.modeltypeid == modeltype_id)
 
+    # Exact-match natural-key lookup for idempotent import. Natural key is
+    # modelnumber + vendor; pair this with ?vendor=<id> to disambiguate.
+    if exactmodelnumber := request.args.get('modelnumber'):
+        query = query.filter(Model.modelnumber == exactmodelnumber)
+
     if search := request.args.get('search'):
         query = query.filter(Model.modelnumber.ilike(f'%{search}%'))
 
@@ -105,6 +111,7 @@ def create_model():
     )
 
     db.session.add(m)
+    apply_import_timestamps(m, data)
     db.session.commit()
 
     return success_response(m.to_dict(), message='Model created', http_code=201)
@@ -132,6 +139,7 @@ def update_model(model_id: int):
         if key in data:
             setattr(m, key, data[key])
 
+    apply_import_timestamps(m, data)
     db.session.commit()
     return success_response(m.to_dict(), message='Model updated')