fix(downloads): a user's download is judged at every connection it makes, not only at the URL typed

The URL guard judged the address the user typed; a public host's 3xx into our
network, a page's formats pointing inside, and a name rebound between the check
and the fetch all got past it, and the stored error told a live internal port
from a dead one. Every non-YouTube download now runs through an egress gate on
the worker's loopback: it resolves each host once, refuses any forbidden answer
before dialling, and connects to the address it vetted. Measured on the worker
image: yt-dlp's proxy reaches the Requests and CurlCFFI handlers, native HLS and
ffmpeg alike. Formats fetched over anything but http(s), which ffmpeg would dial
directly, are dropped, and one factory opens every yt-dlp so none skips it.

backend/app/downloads/backfill.py

@@ -16,7 +16,6 @@ Run once per environment:
 
 from urllib.parse import urlsplit
 
-import yt_dlp
 from sqlalchemy import select, update
 from sqlalchemy.orm import Session
 
@@ -25,7 +24,7 @@ from app.config import settings
 from app.db import SessionLocal
 from app.downloads import adapters
 from app.downloads import edit as editmod
-from app.downloads import formats, service
+from app.downloads import egress, formats, service
 from app.downloads.models import DownloadJob, MediaAsset
 from app.models import DISPLAY_TEXT_MAX
 from app.titles import normalize_title
@@ -43,11 +42,15 @@ def extract_meta(url: str, adapter: adapters.SourceAdapter, asset_id: int) -> di
 def _extract_meta_scoped(url: str, adapter: adapters.SourceAdapter, asset_id: int) -> dict | None:
     for clients in adapter.client_sets():
         opts = formats.build_ydl_opts(
-            {}, "%(id)s.%(ext)s", lambda d: None, extra=adapter.ydl_opts_extra(clients)
+            {},
+            "%(id)s.%(ext)s",
+            lambda d: None,
+            extra=adapter.ydl_opts_extra(clients),
+            egress=adapter.egress_guarded,
         )
         opts["skip_download"] = True
         try:
-            with yt_dlp.YoutubeDL(opts) as ydl:
+            with egress.open_ydl(opts) as ydl:
                 info = ydl.extract_info(url, download=False)
             if info:
                 return info