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/app/(dashboard)/messages/view.tsx

@@ -366,8 +366,18 @@ export function MessagesView({
                             {r.username}
                           </span>
                         )}
-                        {r.channel_id && (
+                        {r.thread_name && (
                           <span className="font-mono text-[10px] text-ink-muted">
+                            ▶ {r.thread_name}
+                          </span>
+                        )}
+                        {!r.thread_name && r.channel_name && (
+                          <span className="font-mono text-[10px] text-ink-muted">
+                            #{r.channel_name}
+                          </span>
+                        )}
+                        {!r.thread_name && !r.channel_name && r.channel_id && (
+                          <span className="font-mono text-[10px] text-ink-faint">
                             #{r.channel_id}
                           </span>
                         )}

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 {