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/frontend/src/lib/api/messages.ts

@@ -1,5 +1,9 @@
 import { orpc } from "@/lib/orpc/client";
-import type { AttachmentRecord, MessageRecord } from "@/lib/types";
+import type {
+  AttachmentRecord,
+  MessageRecord,
+  SemanticSearchResult,
+} from "@/lib/types";
 
 export const messagesApi = {
   list: (
@@ -62,4 +66,11 @@ export const messagesApi = {
     orpc.analysis.search({ q, limit }) as unknown as Promise<{
       results: MessageRecord[];
     }>,
+
+  // Public semantic search over the persistent message archive (Qdrant).
+  semanticSearch: (query: string, limit?: number) =>
+    orpc.messages.semanticSearch({ query, limit }) as unknown as Promise<{
+      results: SemanticSearchResult[];
+      nextCursor: null;
+    }>,
 };