fix: a YouTube stream refused after extraction is the gate, not a network fault

Measured on the isolated test stack the same night, on the very same video ~90 minutes after it was
answering with the bot message: yt-dlp got the player response and picked its formats, and the CDN
then refused the stream itself — "unable to download video data: HTTP Error 403: Forbidden".
Signed out it did that on every attempt; with the owner's cookie attached the video downloaded in
full. So the wall moves between the player response and the stream, and the text it wears there was
costing the download twice.

retriable: the format URLs belong to the client that minted them, so the next client set is a real
second chance — but this text classifies as `network` and nothing in the YouTube marker list matched
it, so the ladder ended the job on attempt 1 and never tried the fallback.

catalog: `youtube.media_forbidden` for the cookie-less case, so the UI stops calling it a network
error when nothing was wrong with the network, and names what actually gets past it. Scoped exactly
as narrowly as the measurement: another host keeps the generic `network` category, and so does a
download that already carried a cookie — telling a signed-in user to sign in is the failure the
catalog exists to avoid. EN + HU copy.

backend/tests/test_adapters.py

@@ -115,6 +115,17 @@ def test_client_sets_and_retriable_are_per_source():
     # The empty-set error of a cookie-incapable client list (see _COOKIE_CAPABLE): retriable, so an
     # admin who configures one still gets the next set tried instead of a hard failure.
     assert YouTubeAdapter().retriable("ERROR: No player clients have been requested") is True
+    # The media fetch refused after a successful extraction: the format URLs belong to the client
+    # that minted them, so the next client set is a real second chance (it used to end the job).
+    assert (
+        YouTubeAdapter().retriable(
+            "ERROR: unable to download video data: HTTP Error 403: Forbidden"
+        )
+        is True
+    )
+    assert (
+        SourceAdapter().retriable("ERROR: unable to download video data: HTTP Error 403") is False
+    )
     # Facebook: a second, Chrome-impersonating attempt only WITH the user's cookie, and only behind
     # the parse wall (without a cookie that wall is the login wall — no disguise gets past it).
     fb = FacebookAdapter()