feat(434): distinct-values search endpoint (text fields, Lucene term enumeration)

Adds GET /api/v1/search/fields/{name}/values?q=&limit= — the backend slice of the
visual rule builder's facet-value typeahead (#434). Enumerates distinct Lucene term
values for a text field via MultiFields.GetTerms + TermsEnum, filtered by a
case-insensitive prefix, limit clamped to [1,50]. 404s when the field is absent from
SearchFieldCatalog or is not type "text". ElasticSearchIndex (the optional external
backend) throws NotSupportedException for this method — its text fields are analyzed,
not keyword-mapped, so a terms aggregation isn't safe to guess at without verifying
against a live cluster.

Regenerated OpenAPI trio (v1.json, v1.d.ts, endpoint-index.md).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/api/generated/v1.d.ts

@@ -1461,6 +1461,9 @@ export interface components {
     "type": null | string;
     "group": null | string;
     "values": null | Array<string>;
+  };
+    "SearchFieldValuesResponseModel": {
+    "values": Array<string>;
   };
     "SearchResultAllItemsResponseModel": {
     "movieIds": Array<number>;