fix(places): submit and link the phase variants

/schools/[place]/[phase] shipped as routes but reached nothing. The sitemap
emitted one URL per registry entry and the registry had no phase dimension, so
~950 pages were absent from every sitemap — and PlaceView did not link them
either, leaving them reachable by nothing at all.

That is the query shape the baseline actually showed: 'primary schools in
beccles', 'secondary schools in brentwood'. Publishing the routes without a
path in meant building for the demand and then hiding from it.

Place now carries phase_urns so the per-phase threshold can be applied without
re-querying, the sitemap emits a variant wherever a phase clears the threshold
on its own, and the API exposes the qualifying phases so the place page links
only variants that exist. Outcodes are excluded: nobody searches 'primary
schools in SW11' and those routes do not exist.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015mWQnpye9F299NVRCCSRvj

backend/tests/test_sitemap.py

@@ -268,3 +268,23 @@ def test_place_urls_carry_no_priority_or_changefreq(place_sitemaps):
     for name in ("places-1.xml", "outcodes-1.xml"):
         assert "<priority>" not in place_sitemaps[name]
         assert "<changefreq>" not in place_sitemaps[name]
+
+
+def test_phase_variants_are_submitted_where_the_phase_clears_the_threshold(place_sitemaps):
+    # "primary schools in beccles" is the query shape the baseline showed, so
+    # each variant is its own page and has to be submitted. Emitting only the
+    # bare place URL left ~950 of them reachable by nothing.
+    xml = place_sitemaps["places-1.xml"]
+    assert "<loc>https://www.schoolcompare.co.uk/schools/brentwood/primary</loc>" in xml
+
+
+def test_a_phase_below_its_own_threshold_is_not_submitted(place_sitemaps):
+    # The fixture is six primaries and no secondaries.
+    xml = place_sitemaps["places-1.xml"]
+    assert "/schools/brentwood/secondary" not in xml
+
+
+def test_outcodes_get_no_phase_variants(place_sitemaps):
+    # Nobody searches "primary schools in CM13"; the routes do not exist.
+    xml = place_sitemaps["outcodes-1.xml"]
+    assert "/primary" not in xml and "/secondary" not in xml