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/frontend/src/lib/types/message.ts

@@ -177,6 +177,8 @@ export interface SemanticSearchResult {
   channel_id: string | null;
   guild_id: string | null;
   thread_id: string | null;
+  channel_name: string | null;
+  thread_name: string | null;
 }
 
 export interface MessageActivityBucket {