feat(integrator): add KS2 re-import via Kestra and backend admin endpoint

- backend: POST /api/admin/reimport-ks2 runs full CSV migration in a thread
- backend/docker-compose: ADMIN_API_KEY env var (default: changeme) so the
  key is stable across restarts and the integrator can call the endpoint
- integrator: sources/ks2.py triggers the backend endpoint (900s timeout)
- integrator: flows/ks2.yml Kestra flow (manual trigger, no schedule)

To re-ingest after a DB wipe: trigger the ks2-reimport flow from the
Kestra UI at http://localhost:8080.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -34,6 +34,7 @@ services:
     environment:
       DATABASE_URL: postgresql://schoolcompare:schoolcompare@db:5432/schoolcompare
       PYTHONUNBUFFERED: 1
+      ADMIN_API_KEY: ${ADMIN_API_KEY:-changeme}
     volumes:
       - ./data:/app/data:ro
     depends_on:
@@ -121,6 +122,8 @@ services:
     environment:
       DATABASE_URL: postgresql://schoolcompare:schoolcompare@db:5432/schoolcompare
       DATA_DIR: /data
+      BACKEND_URL: http://backend:80
+      ADMIN_API_KEY: ${ADMIN_API_KEY:-changeme}
       PYTHONUNBUFFERED: 1
     volumes:
       - ./data:/data