feat(archive): rich metadata in semantic search — username/channel/guild context + guild filter

Archive payload now stores username, channel_id, guild_id, thread_id and the
real message created_at (not embed time). Backend searchArray accepts an
optional guildId and applies a Qdrant payload filter so results can be scoped
to the guild being viewed. API/frontend expose the new fields and the
semantic results card shows who said it, in which channel, and when —
turning bare text blobs into contextual results. Old points fall back to
analyzed_at and omit the new fields gracefully.

services/discord-gateway/src/modules/ai-moderation/qdrantClient.ts

@@ -37,6 +37,14 @@ export interface QdrantVerdictPayload {
   /** Bare content hash (16 hex chars) — enables content-based invalidation
    *  regardless of the (context-scoped) point id. */
   content_hash?: string;
+  // Archive metadata (archiveEmbedder writes these; moderation cache doesn't).
+  // Optional so the same payload shape serves both the moderation cache
+  // collection and the public message archive collection.
+  username?: string;
+  channel_id?: string;
+  guild_id?: string;
+  thread_id?: string | null;
+  created_at?: number;
 }
 
 function baseUrl(): string {