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>

integrator/flows/ks2.yml

@@ -0,0 +1,22 @@
+id: ks2-reimport
+namespace: schoolcompare.data
+description: Re-import KS2 attainment data from bundled CSV files (use after DB wipe)
+
+# No scheduled trigger — run manually from the Kestra UI when needed.
+
+tasks:
+  - id: reimport
+    type: io.kestra.plugin.core.http.Request
+    uri: http://integrator:8001/run/ks2?action=load
+    method: POST
+    allowFailed: false
+    timeout: PT20M   # full migration takes ~10 min; give headroom
+
+errors:
+  - id: notify-failure
+    type: io.kestra.plugin.core.log.Log
+    message: "KS2 re-import FAILED: {{ error.message }}"
+
+retry:
+  maxAttempts: 2
+  delay: PT5M