fix(search): return 422 not 502 on an upstream YouTube failure (B-core-1)

/api/search/youtube raised 502 on an upstream error, but the frontend treats
502/503/504 as a gateway dropout: it auto-retries the GET (a second quota-costed
search) and shows a "connection lost" banner instead of the real message. Every
sibling upstream-error site already returns 422 for exactly this reason. Reserve
5xx for the raw byte-proxies.

backend/app/routes/search.py

@@ -268,7 +268,11 @@ def search_youtube(
             except (YouTubeError, search_scrape.ScrapeError) as exc:
                 if collected:
                     break  # keep what we already gathered
-                raise HTTPException(status_code=502, detail=f"YouTube search failed: {exc}")
+                # 422, not 502: this is a JSON endpoint, and the FE treats 502/503/504 as a gateway
+                # dropout (auto-retry + "connection lost" banner — lib/api/core RETRIABLE_GATEWAY),
+                # which hides the real message and burns a second quota-costed search. Upstream failure
+                # on a JSON endpoint = 422 + message; reserve 5xx for the raw byte-proxies. (B-core-1)
+                raise HTTPException(status_code=422, detail=f"YouTube search failed: {exc}") from exc
 
             # Drop currently-live/upcoming, channel-less, and blocked-channel results.
             candidates = [