fix(downloads): the egress filter keeps what yt-dlp fetches itself, and shows no phase

f4m, ism, a bare m3u8 (native under hls_prefer_native) and the DASH generator fetch through the
proxy, but were dropped as if ffmpeg dialled them. The filter also took the job's postprocessor
hook, twice, and painted every guarded job "processing" before a byte arrived. Tests now hold an
allowed tunnel, the worker's own gate, the single-URL branch, and that the pipeline and the
backfill turn the gate on while YouTube does not.

backend/tests/test_facebook_impersonation.py

@@ -18,7 +18,7 @@ import yt_dlp
 from yt_dlp.networking.exceptions import TransportError
 from yt_dlp.networking.impersonate import ImpersonateTarget
 
-from app.downloads import adapters, pipeline
+from app.downloads import adapters, egress, pipeline
 from app.downloads import errors as dlerrors
 
 _URL = "https://www.facebook.com/reel/806379462330494/?fs=e&fs=e"
@@ -160,6 +160,16 @@ def test_a_cookie_group_post_downloads_the_posts_own_video(ydl, tmp_path):
     # Never the post link: that is where the feed video comes from.
     assert web["extracted"] == [watch, watch]
     assert [o.get("impersonate") for o in seen[1:]] == [None, _CHROME]
+    # The post page read and both attempts all go through the egress gate.
+    assert all(o.get(egress.GUARDED) and o["proxy"] == egress.proxy_url() for o in seen), seen
+
+
+def test_a_youtube_download_skips_the_gate(ydl, tmp_path):
+    seen, outcomes, _ = ydl
+    outcomes += [{"id": "dQw4w9WgXcQ"}]
+    yt = adapters.resolve_adapter("youtube", "dQw4w9WgXcQ")
+    pipeline._extract_with_fallback(1, 1, "https://youtu.be/dQw4w9WgXcQ", {}, tmp_path, yt)
+    assert seen and not any(o.get(egress.GUARDED) or o.get("proxy") for o in seen)
 
 
 def test_a_group_post_without_a_single_video_fails_saying_so(ydl, tmp_path):