fix(ks2): make reimport async with polling to avoid HTTP timeout

The geocoding pass over ~15k schools takes longer than any reasonable
HTTP timeout. New approach:
- POST /api/admin/reimport-ks2 starts migration in background thread,
  returns {"status":"started"} immediately
- GET /api/admin/reimport-ks2/status returns {running, done}
- ks2.py polls status every 30s (max 2h) before returning
- Kestra flow timeout bumped to PT2H

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

backend/app.py

@@ -635,6 +635,9 @@ async def reload_data(
     return {"status": "reloaded"}
 
 
+_reimport_status: dict = {"running": False, "done": False, "error": None}
+
+
 @app.post("/api/admin/reimport-ks2")
 @limiter.limit("2/minute")
 async def reimport_ks2(
@@ -643,19 +646,45 @@ async def reimport_ks2(
     _: bool = Depends(verify_admin_api_key)
 ):
     """
-    Re-run the full KS2 CSV migration (drop + reimport schools and results).
-    Use when the database has been wiped and needs repopulating from the CSV files.
+    Start a full KS2 CSV migration in the background and return immediately.
+    Poll GET /api/admin/reimport-ks2/status to check progress.
     Pass ?geocode=false to skip postcode → lat/lng resolution.
-    Runs synchronously — expect this to take several minutes.
     Requires X-API-Key header with valid admin API key.
     """
-    loop = asyncio.get_event_loop()
-    success = await loop.run_in_executor(None, lambda: run_full_migration(geocode=geocode))
-    if not success:
-        raise HTTPException(status_code=500, detail="Migration failed: no CSV data found in data directory")
-    clear_cache()
-    load_school_data()
-    return {"status": "reimported"}
+    global _reimport_status
+    if _reimport_status["running"]:
+        return {"status": "already_running"}
+
+    _reimport_status = {"running": True, "done": False, "error": None}
+
+    def _run():
+        global _reimport_status
+        try:
+            success = run_full_migration(geocode=geocode)
+            if not success:
+                _reimport_status = {"running": False, "done": False, "error": "No CSV data found"}
+                return
+            clear_cache()
+            load_school_data()
+            _reimport_status = {"running": False, "done": True, "error": None}
+        except Exception as exc:
+            _reimport_status = {"running": False, "done": False, "error": str(exc)}
+
+    import threading
+    threading.Thread(target=_run, daemon=True).start()
+    return {"status": "started"}
+
+
+@app.get("/api/admin/reimport-ks2/status")
+async def reimport_ks2_status(
+    request: Request,
+    _: bool = Depends(verify_admin_api_key)
+):
+    """Poll this endpoint to check reimport progress."""
+    s = _reimport_status
+    if s["error"]:
+        raise HTTPException(status_code=500, detail=s["error"])
+    return {"running": s["running"], "done": s["done"]}
 
 
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