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/src/modules/ai-moderation/verdictToActionFields.ts

@@ -0,0 +1,48 @@
+import type { MessageRecord } from "../message-capture/types.js";
+
+/**
+ * Map a captured message's persisted AI verdict (the `ai_*` columns on
+ * MessageRecord) into the explainability columns of a moderation action.
+ *
+ * This is READ-ONLY structured data — it never changes any enforcement
+ * decision. It exists so the public web view can show *why* a message was
+ * moderated, making GMW's automod transparent instead of a black box.
+ *
+ * All fields are null-safe: manual actions (e.g. command-handler bans) carry
+ * no AI verdict, so they simply store nulls and the UI falls back to the
+ * free-text `reason`.
+ */
+export function verdictToActionFields(message?: MessageRecord | null): {
+  flags: string | null;
+  categories: string | null;
+  severity: string | null;
+  confidence: number | null;
+  score: number | null;
+  evidence: string | null;
+  policy_version: string | null;
+} {
+  if (!message) {
+    return {
+      flags: null,
+      categories: null,
+      severity: null,
+      confidence: null,
+      score: null,
+      evidence: null,
+      policy_version: null,
+    };
+  }
+
+  // ai_moderation_flags / ai_categories are stored as JSON-stringified TEXT
+  // (see messagesAnalysis.buildAIAnalysisSet → stringifyAIList). Pass them
+  // through verbatim so the backend can JSON.parse them back into arrays.
+  return {
+    flags: message.ai_moderation_flags ?? null,
+    categories: message.ai_categories ?? null,
+    severity: message.ai_severity ?? null,
+    confidence: message.ai_confidence ?? null,
+    score: message.ai_moderation_score ?? null,
+    evidence: null, // not persisted on MessageRecord; reserved for future use
+    policy_version: null, // set by caller if a policy version is available
+  };
+}