fix: a YouTube video behind the bot wall is downloadable with cookies

A YouTube link can fail with "Sign in to confirm you're not a bot" while every
other YouTube download works — YouTube gates INDIVIDUAL videos behind a
signed-in session. Measured on prod job 273: all twelve of yt-dlp's player
clients fail on that one video while its neighbours download fine, the bgutil
PO-token sidecar is live and answering, and the newest yt-dlp fails identically.
So the download is not broken by our stack or a stale yt-dlp, and no retry,
client switch or upgrade gets past it. A cookie is the only way in.

The failure copy already sends the user to Settings -> Account -> Download
authentication, but YouTube was not in the cookie host registry, so that
dropdown could not help them: the app asked for something it did not offer.

credentials: YouTube joins SUPPORTED_HOSTS (youtube.com, youtu.be,
youtube-nocookie.com). The Settings dropdown, the SEC-12 host scope and the
private-asset cache salt all derive from the row, so that is the whole wiring.

adapters: a signed-in YouTube download runs only cookie-capable player clients.
Measured against yt-dlp's own INNERTUBE_CLIENTS table: with account cookies
attached yt-dlp REMOVES every client whose SUPPORTS_COOKIES is False, and a set
it empties that way raises "No player clients have been requested" — which is
not a per-client flake it retries past. Our primary set is android_vr, one of
those, so without this the cookie would have failed the download on attempt 1
and never reached the POT-backed web fallback it exists for. A test pins the
client list against the installed yt-dlp so a version bump turns the gate red
instead of silently un-authing every YouTube download, and the empty-set message
joins the retriable markers as the second line of defence.

errors: catalog entries for the wall with and without a cookie, so a user who
has already added cookies is told they were rejected (YouTube rotates exported
ones) rather than to add some. The marker is quote-agnostic: YouTube writes the
apostrophe as U+2019, so "you're not a bot" with a straight quote never matches
the real text. EN + HU copy.

backend/tests/test_download_errors.py

@@ -84,6 +84,49 @@ _REEL = "https://www.facebook.com/reel/1736360027626346/?fs=e&fs=e"
 _GROUP_POST = "https://www.facebook.com/groups/983554479435596/permalink/1755988225525547/?"
 
 
+# Verbatim from prod job 273 (2026-09-15). YouTube writes the apostrophe as U+2019, which is why
+# neither the rule nor the catalog entry may spell out "you’re" with a straight quote.
+_YT_BOT_WALL = (
+    "ERROR: [youtube] 7JPmS_bWjhI: Sign in to confirm you’re not a bot. Use "
+    "--cookies-from-browser or --cookies for the authentication. See  "
+    "https://github.com/yt-dlp/yt-dlp/wiki/FAQ#how-do-i-pass-cookies-to-yt-dlp  for how to manually "
+    "pass cookies."
+)
+_YT = "https://www.youtube.com/watch?v=7JPmS_bWjhI"
+
+
+def test_youtube_bot_wall_is_that_video_own_signed_out_gate():
+    # Measured on prod: all 12 player clients fail on this video while its neighbours download, the
+    # POT sidecar answers, and the newest yt-dlp fails identically — a cookie is the only way in.
+    assert classify(_YT_BOT_WALL) == "login_required"
+    for url in (
+        _YT,
+        "https://youtu.be/7JPmS_bWjhI",
+        "https://www.youtube-nocookie.com/embed/7JPmS_bWjhI",
+        "https://music.youtube.com/watch?v=7JPmS_bWjhI",
+    ):
+        assert dlerrors.diagnose(_YT_BOT_WALL, url=url) == (
+            "login_required",
+            "youtube.bot_wall",
+        ), url
+
+
+def test_youtube_bot_wall_with_a_cookie_is_the_cookie_refused():
+    # Telling a user who already gave a cookie to add one is what this entry exists to avoid.
+    assert dlerrors.diagnose(_YT_BOT_WALL, url=_YT, cookie_attached=True) == (
+        "login_required",
+        "youtube.cookie_rejected",
+    )
+
+
+def test_the_youtube_entries_never_match_another_site():
+    # Host-scoped: the same words about a Facebook link stay the bare category.
+    assert dlerrors.diagnose(
+        "ERROR: [facebook] 1: Sign in to confirm you’re not a bot",
+        url="https://www.facebook.com/reel/1/",
+    ) == ("login_required", None)
+
+
 def test_facebook_cannot_parse_data_is_the_login_wall_without_a_cookie():
     for url in (
         _REEL,