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/config.py

@@ -65,6 +65,10 @@ class Settings(BaseSettings):
     # Optional API key for public reads (channels/videos/playlistItems). When set it is
     # preferred for shared backfill/enrichment so it doesn't depend on a specific user.
     youtube_api_key: str = ""
+    # Optional HTTP(S) proxy for outbound YouTube Data API calls. Set this to send the
+    # scheduler's googleapis traffic through a fixed-IP host (e.g. the VPS over WireGuard) so an
+    # IP-restricted API key keeps working even when this host's public IP is dynamic.
+    youtube_api_proxy: str = ""
     # Daily quota budget (units). Default leaves headroom under the 10,000/day free limit.
     quota_daily_budget: int = 9000
     # Recent-first backfill: how far back to fetch on the first pass per channel.