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/shared/database/schema.ts

@@ -662,6 +662,16 @@ export const pgModerationActionsTable = pgTable(
     error: pgText("error"),
     created_at: pgBigint("created_at", { mode: "number" }).notNull(),
     executed_at: pgBigint("executed_at", { mode: "number" }),
+    // ── Explainability (structured verdict; surfaced read-only to public web) ──
+    flags: pgText("flags"), // JSON array of string flags, e.g. ["sara_agama","vulgar"]
+    categories: pgText("categories"), // JSON array of category strings
+    severity: pgText("severity", {
+      enum: ["none", "low", "medium", "high", "critical"],
+    }),
+    confidence: pgReal("confidence"), // 0..1
+    score: pgReal("score"), // 0..1 raw model score
+    evidence: pgText("evidence"), // JSON array of short quoted snippets
+    policy_version: pgText("policy_version"), // rules.ts policy version string
   },
   (table) => ({
     messageIdIdx: pgIndex("idx_moderation_actions_message_id").on(