Fix: Update all old-style imports inside packages to new paths

65 files in klausur-service packages + 3 in backend-lehrer packages
had stale imports referencing deleted shim modules.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

klausur-service/backend/admin/api.py

@@ -7,7 +7,7 @@ This module was split into:
   - admin_templates.py  (Legal templates ingestion, search)
 
 The `router` object is assembled here by including all sub-routers.
-Importers that did `from admin_api import router` continue to work.
+Importers that did `from admin.api import router` continue to work.
 """
 
 from fastapi import APIRouter