feat(gmw): moderation explainability + semantic message search

- Persist structured verdict (flags/severity/confidence/evidence) on
  moderation_actions so the public web can show WHY a message was moderated.
- Add a persistent Qdrant archive collection (gmw_message_archive); embed
  every captured message at capture time (fire-and-forget, best-effort).
- Public semantic search over the archive (backend oRPC + FE toggle on the
  messages view). Both features are read-only/public and fully automatic.

Migration: 0015_add_moderation_explainability.sql

services/backend/src/orpc/router.ts

@@ -16,7 +16,10 @@ import {
   skip,
   stop,
 } from "../modules/media/media.service";
-import { messageQuerySchema } from "../modules/messages/messages.schema";
+import {
+  messageQuerySchema,
+  semanticSearchSchema,
+} from "../modules/messages/messages.schema";
 import { messagesService } from "../modules/messages/messages.service";
 import { moderationService } from "../modules/moderation/moderation.service";
 import { recordingsService } from "../modules/recordings/recordings.service";
@@ -136,6 +139,10 @@ const messagesRouter = {
       );
       return { results: rows, limit: input.limit, cursor: null };
     }),
+  // Public, read-only semantic search over the message archive.
+  semanticSearch: os
+    .input(semanticSearchSchema)
+    .handler(({ input }) => messagesService.semanticSearch(input)),
 };
 
 // ── Moderation ───────────────────────────────────────────────────