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/modules/messages/messages.schema.ts

@@ -41,3 +41,11 @@ export const messageUpdateSchema = z.object({
 export type MessageQuery = z.infer<typeof messageQuerySchema>;
 export type MessageCreate = z.infer<typeof messageCreateSchema>;
 export type MessageUpdate = z.infer<typeof messageUpdateSchema>;
+
+export const semanticSearchSchema = z.object({
+  query: z.string().min(1).max(500),
+  limit: z.coerce.number().int().positive().max(50).default(10),
+  guildId: z.string().optional(),
+});
+
+export type SemanticSearchQuery = z.infer<typeof semanticSearchSchema>;