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_titles.py

@@ -10,7 +10,7 @@ from sqlalchemy import select, update
 
 from app.config import settings
 from app.db import SessionLocal
-from app.downloads import adapters, backfill, pipeline
+from app.downloads import adapters, backfill, egress, pipeline
 from app.downloads.models import DownloadJob, MediaAsset
 
 _URL = "https://www.facebook.com/reel/1860682468422402/"
@@ -61,6 +61,7 @@ def test_a_placeholder_with_nothing_behind_it_takes_the_signed_out_pages_text(mo
     assert len(seen) == 1
     assert seen[0]["skip_download"] is True
     assert not seen[0].get("cookiefile")
+    assert seen[0][egress.GUARDED] is True and seen[0]["proxy"] == egress.proxy_url()
 
 
 def test_a_real_title_reads_nothing_more(monkeypatch):