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.

backend/app/sysconfig.py

@@ -50,6 +50,8 @@ SPECS: tuple[ConfigSpec, ...] = (
     ConfigSpec("autotag_title_sample", "int", "batch", "autotag_title_sample", min=1, max=1_000),
     # --- YouTube Data API key (secret; optional — public reads prefer it over OAuth) ---
     ConfigSpec("youtube_api_key", "str", "youtube", "youtube_api_key", secret=True),
+    # Optional egress proxy for YouTube API calls (fixed-IP host for an IP-restricted key).
+    ConfigSpec("youtube_api_proxy", "str", "youtube", "youtube_api_proxy"),
     # --- Google OAuth client (Sign in with Google; both encrypted, env fallback). Set from the
     #     install wizard / Configuration page so no restart is needed when the creds change. ---
     ConfigSpec("google_client_id", "str", "google", "google_client_id", secret=True),

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