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_formats.py

@@ -15,16 +15,20 @@ from app.downloads import formats
 
 
 def _opts(spec):
-    return formats.build_ydl_opts(spec, "/tmp/%(id)s.%(ext)s", lambda d: None)
+    return formats.build_ydl_opts(spec, "/tmp/%(id)s.%(ext)s", lambda d: None, egress=False)
 
 
 def test_adapter_extra_opts_are_merged_in():
     # The SourceAdapter contributes source-specific opts (e.g. YouTube's extractor_args) via `extra`.
     extra = {"extractor_args": {"youtube": {"player_client": ["web"]}}}
-    opts = formats.build_ydl_opts({}, "/tmp/%(id)s.%(ext)s", lambda d: None, extra=extra)
+    opts = formats.build_ydl_opts(
+        {}, "/tmp/%(id)s.%(ext)s", lambda d: None, extra=extra, egress=False
+    )
     assert opts["extractor_args"]["youtube"]["player_client"] == ["web"]
     # No extra → no extractor_args (a generic source contributes nothing).
-    assert "extractor_args" not in formats.build_ydl_opts({}, "/tmp/x", lambda d: None)
+    assert "extractor_args" not in formats.build_ydl_opts(
+        {}, "/tmp/x", lambda d: None, egress=False
+    )
 
 
 def test_mp4_default_selects_avc1_with_progressive_fallback():
@@ -113,7 +117,7 @@ def test_a_playlist_url_can_only_yield_one_item():
 
 
 def test_the_remaining_quota_becomes_a_hard_size_ceiling():
-    opts = formats.build_ydl_opts({}, "/tmp/x", lambda d: None, max_bytes=5_000_000)
+    opts = formats.build_ydl_opts({}, "/tmp/x", lambda d: None, max_bytes=5_000_000, egress=False)
     assert opts["max_filesize"] == 5_000_000
 
 
@@ -122,7 +126,7 @@ def test_no_budget_means_no_size_filter():
     # enqueue, not an impossible yt-dlp filter to express here.
     for budget in (None, 0, -1):
         assert "max_filesize" not in formats.build_ydl_opts(
-            {}, "/tmp/x", lambda d: None, max_bytes=budget
+            {}, "/tmp/x", lambda d: None, max_bytes=budget, egress=False
         )
 
 
@@ -130,7 +134,12 @@ def test_a_source_adapter_cannot_raise_the_size_ceiling():
     # The adapter merge happens after max_filesize is set, so this pins that no adapter option can
     # widen a storage cap — the mirror of the cookiefile rule, in the opposite direction.
     opts = formats.build_ydl_opts(
-        {}, "/tmp/x", lambda d: None, extra={"max_filesize": 999_999_999}, max_bytes=5_000_000
+        {},
+        "/tmp/x",
+        lambda d: None,
+        extra={"max_filesize": 999_999_999},
+        max_bytes=5_000_000,
+        egress=False,
     )
     assert opts["max_filesize"] == 5_000_000
 
@@ -197,7 +206,7 @@ def test_yt_dlp_refuses_a_file_over_the_cap_without_raising_and_the_watch_sees_i
         for cap, refused in ((1_000, True), (10_000_000, False)):
             out = tmp_path / f"out-{cap}"
             opts = formats.build_ydl_opts(
-                {}, str(out / "%(id)s.%(ext)s"), lambda d: None, max_bytes=cap
+                {}, str(out / "%(id)s.%(ext)s"), lambda d: None, max_bytes=cap, egress=False
             )
             with formats.watching_size_cap() as over, yt_dlp.YoutubeDL(opts) as ydl:
                 ydl.extract_info(url, download=True)