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/measuringtools/api/routes.py

@@ -19,7 +19,7 @@ from shopdb.api import (
     db, Asset, AssetType, AuditLog,
     success_response, error_response, paginated_response, ErrorCodes,
     get_pagination_params, paginate_query,
-    require_permission,
+    require_permission, apply_import_timestamps,
 )
 
 from ..models import MeasuringTool, MeasuringToolType, derive_status, STATUS_COLORS
@@ -165,6 +165,9 @@ def list_tools():
 
     if request.args.get('active', 'true').lower() != 'false':
         query = query.filter(Asset.isactive == True)
+    # Exact-match natural-key lookup for idempotent import (asset number).
+    if exactassetnumber := request.args.get('assetnumber'):
+        query = query.filter(Asset.assetnumber == exactassetnumber)
     if search := request.args.get('search'):
         query = query.filter(db.or_(
             Asset.assetnumber.ilike(f'%{search}%'),
@@ -270,6 +273,7 @@ def create_tool():
     db.session.add(tool)
     db.session.flush()
 
+    apply_import_timestamps(asset, data)
     AuditLog.log('created', 'MeasuringTool', entityid=tool.measuringtoolid,
                  entityname=asset.assetnumber)
     db.session.commit()
@@ -321,6 +325,7 @@ def update_tool(tool_id: int):
     if changes:
         AuditLog.log('updated', 'MeasuringTool', entityid=tool.measuringtoolid,
                      entityname=asset.assetnumber, changes=changes)
+    apply_import_timestamps(asset, data)
     db.session.commit()
     return success_response(_merged(tool), message='Measuring tool updated')