feat(downloads): per-user, per-host yt-dlp cookie/auth support (S6)

Each user stores their own login cookies (Netscape cookies.txt) for an
auth-walled download host (Facebook, Instagram, X, TikTok, Reddit) in
Settings; a download of an auth-walled link on that host is retried with
the acting user's cookie.

Two load-bearing security controls, both in downloads.credentials as the
single source of truth so enqueue and the worker can't drift:
- cache privacy: a cookie-authed download salts its MediaAsset format_sig
  with the user id, so the authed asset is private and never cache-served
  to another user. The worker attaches a cookie ONLY when the asset it is
  downloading into IS that private, salted identity.
- host scope (SEC-12): a cookie is resolved only for a URL whose netloc
  the credential owns (netloc-scoped _host_is); a look-alike/unrelated
  host gets none.

cookies_enc is Fernet-encrypted at rest, only decrypted in-worker into a
0600 file under staging/.auth/ (survives the retry wipe + _produced_file
scan) and unlinked in finally. The blob is never returned by a route,
logged, audited, or written to a job's error/snapshot. cookiefile is set
after the adapter's extra merge so an adapter can't clobber it. Settings
UI is write-only (host + set/updated only, never echoed). New
download_credentials table + migration 0074; per-user routes; HU+EN i18n.

Tests: host-scope, cache-privacy (no cross-user serve), encrypt-at-rest,
write-only API, worker attach-gate; FE dom test for the write-only field.

backend/app/downloads/formats.py

@@ -98,12 +98,17 @@ def build_ydl_opts(
     outtmpl: str,
     progress_hook,
     extra: dict | None = None,
+    cookiefile: str | None = None,
 ) -> dict:
     """Assemble yt-dlp options for a single download to `outtmpl` (a full path template).
 
     `extra` is source-specific options contributed by the SourceAdapter (e.g. YouTube's
     `extractor_args` with the player-client set + bgutil PO-token base_url); it is merged onto the
-    generic options below. Everything here is source-agnostic."""
+    generic options below. Everything here is source-agnostic.
+
+    `cookiefile` is the acting user's per-host cookies.txt path (S6), attached ONLY by the worker
+    after the host-scope + private-asset gate in `downloads.credentials`. It is set AFTER the `extra`
+    merge so a source adapter can never clobber it."""
     s = normalize(spec)
     h = s["max_height"]
     opts: dict = {
@@ -132,6 +137,11 @@ def build_ydl_opts(
     if extra:
         opts.update(extra)
 
+    # The acting user's per-host cookie (S6). AFTER the adapter merge on purpose: an adapter must not
+    # be able to override the authenticated session the worker resolved for this specific user.
+    if cookiefile:
+        opts["cookiefile"] = cookiefile
+
     if s["mode"] == "a":
         opts["format"] = "bestaudio/best"
         opts["postprocessors"].append(