feat(places): name every authority a place sits in

SW19 is mostly Merton but partly Wandsworth, and the page said only Merton.
The cause was one field doing two jobs: _parent_authority takes the modal
authority, which is right for a 301 target and wrong as a statement about
where a place is.

This is not a corner case. A quarter of viable outcodes (425 of 1,760) and a
third of viable towns (263 of 783) cross an authority boundary — Bedford the
town spans Bedford and Central Bedfordshire.

Place now carries `authorities`, every authority holding at least a tenth of
the schools and at least two of them, largest first. parent_authority stays
single and unchanged, because a redirect still needs one target.

The share threshold exists because GIAS carries postcode errors: EN6 lists two
Shropshire schools among fourteen in Hertfordshire, and a bare "any authority
present" rule would print those as though they were real. A place too small or
too fragmented to clear the threshold still names its largest, so the page
never goes silent about where it is.

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

backend/app.py

@@ -1232,6 +1232,13 @@ async def get_place(request: Request, kind: str, slug: str,
         "place": {"kind": place.kind, "slug": place.slug, "name": place.name,
                   "count": len(place.urns),
                   "parent_authority": place.parent_authority,
+                  # Every authority the place meaningfully sits in. SW19 is
+                  # mostly Merton but partly Wandsworth; naming one asserts
+                  # something false.
+                  "authorities": [
+                      {"name": name, "slug": _slugify(name), "count": n}
+                      for name, n in place.authorities
+                  ],
                   # Only phases that clear the threshold, so the page links
                   # variants that exist rather than 404s.
                   "phases": [ph for ph in ("primary", "secondary")