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/app/config.py

@@ -155,6 +155,9 @@ class Settings(BaseSettings):
     # per-client responses are flaky, and android_vr can get bot-flagged under heavy load) the
     # worker retries with the FALLBACK set — the POT-capable web clients (web needs the token +
     # Deno) plus android for audio. Both admin-tunable when YouTube shifts what works.
+    # A SIGNED-IN download (the user added YouTube cookies) keeps only the cookie-capable clients
+    # of these two sets — yt-dlp drops the rest, so android_vr / android / ios never run there.
+    # See adapters.YouTubeAdapter._COOKIE_CAPABLE.
     download_player_clients: str = "android_vr"
     download_player_clients_fallback: str = "web_safari,web,android"