feat: wire embeds into all message query and creation paths

Integrate embed infrastructure into the complete message flow:
- messages.ts: batch-fetch embeds in GET, resolve on POST, re-resolve on PATCH
- dm.ts: same pattern for DM messages with isDm=true
- search.ts: include embeds in all 4 search/around endpoints
- events.ts: embed resolution in WS message create/edit for both space and DM
- Fix embedClassifier.ts type errors (regex match undefined → null)
- Add embeds: [] to all inline MessageWithUser/DmMessageWithUser constructions
- Add embeds: [] to chatStore optimistic message

packages/server/src/utils/embedClassifier.ts

@@ -25,15 +25,15 @@ function extractYouTubeId(url: URL): string | null {
 
     // /shorts/ID
     const shortsMatch = path.match(/^\/shorts\/([A-Za-z0-9_-]+)/);
-    if (shortsMatch) return shortsMatch[1];
+    if (shortsMatch) return shortsMatch[1] ?? null;
 
     // /embed/ID
     const embedMatch = path.match(/^\/embed\/([A-Za-z0-9_-]+)/);
-    if (embedMatch) return embedMatch[1];
+    if (embedMatch) return embedMatch[1] ?? null;
 
     // /v/ID (legacy)
     const vMatch = path.match(/^\/v\/([A-Za-z0-9_-]+)/);
-    if (vMatch) return vMatch[1];
+    if (vMatch) return vMatch[1] ?? null;
   }
 
   return null;