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/qdrant.ts

@@ -0,0 +1,95 @@
+import { config } from "@/shared/config/index.js";
+import { createChildLogger } from "@/shared/logger/index.js";
+
+const logger = createChildLogger("messages-qdrant");
+
+export interface ArchiveHit {
+  score: number;
+  payload: {
+    text: string;
+    content_hash?: string;
+    analyzed_at: number;
+    expires_at: number;
+  };
+}
+
+function baseUrl(): string {
+  return (config.QDRANT_URL ?? "http://100.121.180.82:6333").replace(
+    /\/+$/,
+    "",
+  );
+}
+
+function headers(): Record<string, string> {
+  const h: Record<string, string> = { "Content-Type": "application/json" };
+  if (config.QDRANT_API_KEY) h["api-key"] = config.QDRANT_API_KEY;
+  return h;
+}
+
+export const ARCHIVE_COLLECTION =
+  config.QDRANT_ARCHIVE_COLLECTION ?? "gmw_message_archive";
+
+async function request(
+  method: string,
+  path: string,
+  body?: unknown,
+  timeoutMs = 10_000,
+): Promise<unknown> {
+  const controller = new AbortController();
+  const timer = setTimeout(() => controller.abort(), timeoutMs);
+  try {
+    const res = await fetch(`${baseUrl()}${path}`, {
+      method,
+      headers: headers(),
+      body: body === undefined ? undefined : JSON.stringify(body),
+      signal: controller.signal,
+    });
+    const text = await res.text();
+    if (!res.ok) {
+      throw new Error(
+        `Qdrant ${method} ${path} -> ${res.status}: ${text.slice(0, 200)}`,
+      );
+    }
+    return text ? JSON.parse(text) : null;
+  } finally {
+    clearTimeout(timer);
+  }
+}
+
+/** Search the archive collection for the nearest vectors to `vector`. */
+export async function searchArchive(
+  vector: number[],
+  limit: number,
+  scoreThreshold: number,
+): Promise<ArchiveHit[]> {
+  if (!config.QDRANT_URL) return [];
+  try {
+    const json = (await request(
+      "POST",
+      `/collections/${ARCHIVE_COLLECTION}/points/search`,
+      {
+        vector,
+        limit,
+        score_threshold: scoreThreshold,
+        with_payload: true,
+      },
+    )) as {
+      result?: Array<{
+        score?: number;
+        payload?: ArchiveHit["payload"];
+      }>;
+    };
+    return (json.result ?? [])
+      .filter((h) => h.payload?.text)
+      .map((h) => ({
+        score: h.score ?? 0,
+        payload: h.payload as ArchiveHit["payload"],
+      }));
+  } catch (error) {
+    logger.warn(
+      { error: error instanceof Error ? error.message : String(error) },
+      "archive search failed",
+    );
+    return [];
+  }
+}