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/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),