fix(api): publish a validated dataset, and stop truncating search

Reload cleared the caches first and rebuilt afterwards, so any failure
left the API serving nothing, and requests arriving mid-reload saw a
half-swapped state. It now builds and validates the replacement frames,
place registry, reverse index and sitemaps off the request loop, then
publishes them in one synchronous step under a lock. A failed reload
returns 503 and keeps the previous data. Sitemap regeneration takes the
same path rather than clearing the live registry up front.

Typesense search returned at most one page of hits and used an empty
list for both "no matches" and "search is down", so a genuine empty
result silently fell back to substring matching. It now pages through
every candidate and returns None only on failure; the fallback matches
literally, since a query containing regex metacharacters used to throw.

Empty datasets answer 503 rather than 200-with-nothing or a misleading
404, so callers can tell an outage from an absent school. Adds
/api/release, which reports the build identity baked into the image.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/tests/test_publication.py

@@ -0,0 +1,68 @@
+"""Publication must preserve the current dataset until every replacement is ready."""
+import asyncio
+import pandas as pd
+import pytest
+from fastapi.testclient import TestClient
+from backend import app as api, data_loader
+from backend.tests.test_sixth_form_flag import _schools_df
+
+
+@pytest.fixture
+def client(monkeypatch):
+    old = _schools_df()
+    monkeypatch.setattr(data_loader, '_df_cache', old)
+    monkeypatch.setattr(data_loader, '_df_latest_cache', old)
+    monkeypatch.setattr(api, '_place_registry', {'old': 'registry'})
+    monkeypatch.setattr(api, '_place_index', {'old': 'index'})
+    monkeypatch.setattr(api, '_place_index_source', api._place_registry)
+    monkeypatch.setattr(api, '_sitemaps', {'old.xml': 'old sitemap'})
+    monkeypatch.setattr(api, '_publication_lock', asyncio.Lock())
+    monkeypatch.setattr(api.limiter, 'enabled', False)
+    api.app.dependency_overrides[api.verify_admin_api_key] = lambda: True
+    yield TestClient(api.app, raise_server_exceptions=False)
+    api.app.dependency_overrides.clear()
+
+
+def state():
+    return (data_loader._df_cache, data_loader._df_latest_cache, api._place_registry,
+            api._place_index, api._place_index_source, api._sitemaps)
+
+
+@pytest.mark.parametrize('failure', ['empty', 'database', 'sitemap', 'duplicate'])
+def test_failed_reload_preserves_every_published_object(client, monkeypatch, failure):
+    before = state()
+    df = _schools_df()
+    if failure == 'empty':
+        df = pd.DataFrame()
+    if failure == 'duplicate':
+        df = pd.concat([df, df.iloc[:1]], ignore_index=True)
+    def load():
+        if failure == 'database':
+            raise RuntimeError('database unavailable')
+        return df
+    monkeypatch.setattr(api, 'load_school_data_as_dataframe', load)
+    if failure == 'sitemap':
+        monkeypatch.setattr(api, 'build_sitemaps', lambda *args: (_ for _ in ()).throw(RuntimeError('bad XML')))
+    response = client.post('/api/admin/reload')
+    assert response.status_code == 503
+    assert all(a is b for a, b in zip(before, state()))
+
+
+def test_success_publishes_school_data_places_and_sitemaps(client, monkeypatch):
+    df = _schools_df()
+    df.loc[0, 'school_name'] = 'Replacement School'
+    monkeypatch.setattr(api, 'load_school_data_as_dataframe', lambda: df)
+    response = client.post('/api/admin/reload')
+    assert response.status_code == 200
+    assert data_loader.load_school_data() is df
+    assert data_loader.load_latest_school_data().iloc[0].school_name == 'Replacement School'
+    assert api._place_index_source is api._place_registry
+    assert 'old.xml' not in api._sitemaps
+    assert 'replacement-school' in api._sitemaps['schools-1.xml']
+
+
+def test_failed_sitemap_regeneration_keeps_existing_publication(client, monkeypatch):
+    before = state()
+    monkeypatch.setattr(api, 'build_sitemaps', lambda *args: (_ for _ in ()).throw(RuntimeError('bad XML')))
+    assert client.post('/api/admin/regenerate-sitemap').status_code == 503
+    assert all(a is b for a, b in zip(before, state()))