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/assets.py

@@ -15,6 +15,7 @@ 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
 
 assets_bp = Blueprint('assets', __name__)
 
@@ -470,6 +471,7 @@ def create_asset():
     )
 
     db.session.add(asset)
+    apply_import_timestamps(asset, data)
     db.session.commit()
 
     return success_response(asset.to_dict(), message='Asset created', http_code=201)
@@ -512,6 +514,7 @@ def update_asset(asset_id: int):
         if key in data:
             setattr(asset, key, data[key])
 
+    apply_import_timestamps(asset, data)
     db.session.commit()
     return success_response(asset.to_dict(), message='Asset updated')
 
@@ -657,6 +660,7 @@ def create_asset_relationship():
     )
 
     db.session.add(rel)
+    apply_import_timestamps(rel, data)
     db.session.commit()
 
     return success_response(rel.to_dict(), message='Relationship created', http_code=201)