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/app.py

@@ -209,12 +209,27 @@ def _place_url(place) -> str:
 
 
 def _place_sitemap_rows(kinds: tuple[str, ...]) -> list[str]:
-    return [
-        _url_element(BASE_URL + _place_url(p))
-        for p in sorted(get_place_registry().values(),
-                        key=lambda p: (p.kind, p.slug))
-        if p.kind in kinds
-    ]
+    """A <url> per place, plus a phase variant wherever that phase clears the
+    threshold on its own.
+
+    Phase is part of the query — "primary schools in beccles" — so each
+    variant is its own indexable page. Submitting only the bare place URL left
+    ~950 of them reachable by nothing: absent from every sitemap, and not
+    linked from the place page either.
+    """
+    rows: list[str] = []
+    for p in sorted(get_place_registry().values(), key=lambda p: (p.kind, p.slug)):
+        if p.kind not in kinds:
+            continue
+        rows.append(_url_element(BASE_URL + _place_url(p)))
+        # Outcodes carry no phase variants: nobody searches "primary schools
+        # in SW11", so the routes do not exist to submit.
+        if p.kind == "outcode":
+            continue
+        for phase in ("primary", "secondary"):
+            if p.publishes_phase(phase):
+                rows.append(_url_element(f"{BASE_URL}{_place_url(p)}/{phase}"))
+    return rows
 
 
 def build_sitemaps() -> dict[str, str]:
@@ -1216,7 +1231,11 @@ async def get_place(request: Request, kind: str, slug: str,
     return {
         "place": {"kind": place.kind, "slug": place.slug, "name": place.name,
                   "count": len(place.urns),
-                  "parent_authority": place.parent_authority},
+                  "parent_authority": place.parent_authority,
+                  # Only phases that clear the threshold, so the page links
+                  # variants that exist rather than 404s.
+                  "phases": [ph for ph in ("primary", "secondary")
+                             if place.publishes_phase(ph)]},
         "schools": clean_for_json(rows[cols]),
         "averages": averages,
     }