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

nextjs-app/lib/places.ts

@@ -18,8 +18,19 @@ export interface PlaceSummary {
   phases?: string[];
 }
 
+export interface PlaceAuthority {
+  name: string;
+  slug: string;
+  count: number;
+}
+
 export interface PlaceDetail {
-  place: PlaceSummary & { parent_authority: string | null };
+  place: PlaceSummary & {
+    parent_authority: string | null;
+    /** Every authority the place meaningfully sits in, largest first. SW19 is
+     *  mostly Merton but partly Wandsworth. */
+    authorities?: PlaceAuthority[];
+  };
   schools: School[];
   averages: {
     rwm_expected_pct: number | null;