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/tests/test_ytdlp_logging.py

@@ -216,7 +216,7 @@ def test_a_line_with_no_url_is_left_alone(sink):
 
 
 def test_the_download_options_carry_the_logger():
-    opts = formats.build_ydl_opts({}, "%(id)s.%(ext)s", lambda d: None)
+    opts = formats.build_ydl_opts({}, "%(id)s.%(ext)s", lambda d: None, egress=False)
 
     assert opts["logger"] is formats._YDL_LOG
 
@@ -227,7 +227,7 @@ def test_a_source_adapter_cannot_silence_it():
     adapter must not be able to take away the one account of what went wrong that a failed job
     leaves behind."""
     opts = formats.build_ydl_opts(
-        {}, "%(id)s.%(ext)s", lambda d: None, extra={"logger": None, "quiet": False}
+        {}, "%(id)s.%(ext)s", lambda d: None, extra={"logger": None, "quiet": False}, egress=False
     )
 
     assert opts["logger"] is formats._YDL_LOG