feat(youtube): optional egress proxy for YouTube API calls

New youtube_api_proxy setting (env fallback + admin Config UI, env YOUTUBE_API_PROXY):
when set, the YouTubeClient routes all its httpx traffic through that HTTP(S) proxy.
Lets a dynamic-IP host send API calls through a fixed-IP host (e.g. the server over
private tunnel) so an IP-restricted API key keeps working. Empty = direct.

backend/app/youtube/client.py

@@ -44,7 +44,10 @@ class YouTubeClient:
     def __init__(self, db, user: User):
         self.db = db
         self.user = user
-        self._http = httpx.Client(timeout=30.0)
+        # Optionally route all YouTube traffic through a fixed-IP egress proxy (e.g. the VPS
+        # over WireGuard) so an IP-restricted API key keeps working from a dynamic-IP host.
+        proxy = sysconfig.get_str(db, "youtube_api_proxy") or None
+        self._http = httpx.Client(timeout=30.0, proxy=proxy)
 
     def __enter__(self) -> "YouTubeClient":
         return self