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/knowledgebase/api/routes.py

@@ -16,7 +16,7 @@ from shopdb.api import (
 
 from ..models import KnowledgeBase
 
-from shopdb.api import require_permission, require_role
+from shopdb.api import require_permission, require_role, apply_import_timestamps
 
 knowledgebase_bp = Blueprint('knowledgebase', __name__)
 
@@ -42,6 +42,13 @@ def list_articles():
     if appid := request.args.get('appid'):
         query = query.filter(KnowledgeBase.appid == int(appid))
 
+    # Exact-match natural-key lookups for idempotent import. linkurl is the
+    # stable natural key; shortdescription (the title) is offered as a fallback.
+    if exactlinkurl := request.args.get('linkurl'):
+        query = query.filter(KnowledgeBase.linkurl == exactlinkurl)
+    if exacttitle := request.args.get('shortdescription'):
+        query = query.filter(KnowledgeBase.shortdescription == exacttitle)
+
     # Sort options
     sort = request.args.get('sort', 'clicks')
     order = request.args.get('order', 'desc')
@@ -165,6 +172,7 @@ def create_article():
     )
 
     db.session.add(article)
+    apply_import_timestamps(article, data)
     db.session.commit()
 
     return success_response(article.to_dict(), message='Article created', http_code=201)
@@ -195,6 +203,7 @@ def update_article(link_id: int):
         if key in data:
             setattr(article, key, data[key])
 
+    apply_import_timestamps(article, data)
     db.session.commit()
     return success_response(article.to_dict(), message='Article updated')