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/embed.ts

@@ -0,0 +1,43 @@
+import { config } from "@/shared/config/index.js";
+import { createChildLogger } from "@/shared/logger/index.js";
+
+const logger = createChildLogger("messages-embed");
+
+/**
+ * Embed a search query with the configured OpenAI-compatible embedding model.
+ * Uses raw fetch (the backend has no openai SDK dependency) and returns null
+ * when embeddings are not configured (search unavailable).
+ *
+ * encoding_format: "float" is REQUIRED — Nvidia-backed models reject base64.
+ */
+export async function embedQuery(text: string): Promise<number[] | null> {
+  if (!config.AI_LLM_API_KEY || !config.AI_LLM_EMBEDDING_MODEL) return null;
+  try {
+    const res = await fetch(`${config.AI_LLM_BASE_URL}/embeddings`, {
+      method: "POST",
+      headers: {
+        "Content-Type": "application/json",
+        Authorization: `Bearer ${config.AI_LLM_API_KEY}`,
+      },
+      body: JSON.stringify({
+        model: config.AI_LLM_EMBEDDING_MODEL,
+        input: text,
+        encoding_format: "float",
+      }),
+    });
+    if (!res.ok) {
+      logger.warn({ status: res.status }, "query embed HTTP error");
+      return null;
+    }
+    const json = (await res.json()) as {
+      data?: Array<{ embedding?: number[] }>;
+    };
+    return json.data?.[0]?.embedding ?? null;
+  } catch (error) {
+    logger.warn(
+      { error: error instanceof Error ? error.message : String(error) },
+      "query embed failed",
+    );
+    return null;
+  }
+}