feat(archive): show real channel/thread names in semantic search UI

Gateway archive embedder now parses metadata.channel.{channelName,threadName}
from each message and stores channel_name/thread_name in the Qdrant payload.
Backend exposes them; the semantic results card renders the thread name (or
channel name) instead of a raw #snowflake, with the ID as a last-resort
fallback for legacy points. Matches the message feed's channel-label logic
(getMessageChannelLabel).

services/backend/src/modules/messages/qdrant.ts

@@ -14,6 +14,8 @@ export interface ArchiveHit {
     channel_id?: string;
     guild_id?: string;
     thread_id?: string | null;
+    channel_name?: string | null;
+    thread_name?: string | null;
     created_at?: number;
   };
 }