fix(api): surface YouTube action failures as 422, not 502

Same fix as the subscribe reconcile, applied to the remaining endpoints that
wrapped a YouTubeError in a 502: video lookup (feed) and playlist refresh,
push-plan, sync, and delete. A 502 is treated by the client as a transient
"connection lost" blip, so a real, explainable YouTube failure showed the wrong
message. 422 carries the clear detail to the error dialog instead.

backend/app/routes/feed.py

@@ -552,7 +552,7 @@ def get_video_detail(
         with quota.attribute(user.id, "video_lookup"), YouTubeClient(db, user) as yt:
             items = yt.get_videos([video_id])
     except YouTubeError as exc:
-        raise HTTPException(status_code=502, detail=f"YouTube lookup failed: {exc}")
+        raise HTTPException(status_code=422, detail=f"YouTube lookup failed: {exc}")
     if not items:
         raise HTTPException(status_code=404, detail="Unknown video")