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/computers/api/routes.py

@@ -7,7 +7,7 @@ from shopdb.api import db, Asset, AssetType, OperatingSystem, Application, AppVe
 
 from ..models import Computer, ComputerType, ComputerInstalledApp, AccessProtocol, ComputerAccess
 
-from shopdb.api import require_permission, require_role
+from shopdb.api import require_permission, require_role, apply_import_timestamps
 
 computers_bp = Blueprint('computers', __name__)
 
@@ -323,6 +323,10 @@ def list_computers():
     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)
+
     # Search filter
     if search := request.args.get('search'):
         query = query.filter(
@@ -551,6 +555,9 @@ def create_computer():
     # Remote-access protocols
     _sync_access_methods(comp, data)
 
+    # Preserve legacy timestamps in import mode (no-op otherwise)
+    apply_import_timestamps(asset, data)
+
     # Audit log
     AuditLog.log('created', 'Computer', entityid=comp.computerid,
                  entityname=data.get('hostname') or data['assetnumber'])
@@ -655,6 +662,7 @@ def update_computer(computer_id: int):
         AuditLog.log('updated', 'Computer', entityid=comp.computerid,
                      entityname=comp.hostname or asset.assetnumber, changes=changes)
 
+    apply_import_timestamps(asset, data)
     db.session.commit()
 
     result = asset.to_dict()