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/discord-gateway/drizzle/migrations/meta/_journal.json

@@ -106,6 +106,13 @@
       "when": 1785621600000,
       "tag": "0014_add_term_glossary_cache",
       "breakpoints": true
+    },
+    {
+      "idx": 15,
+      "version": "7",
+      "when": 1787184000000,
+      "tag": "0015_add_moderation_explainability",
+      "breakpoints": true
     }
   ]
 }