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/audit.py

@@ -44,6 +44,9 @@ class AuditAction:
     SYNC_RESUME = "sync_resume"
     # Scheduler job outcome / run-summary (actor_id set = a manual "run now"; NULL = automatic)
     JOB_RUN = "job_run"
+    # Per-user download cookie/auth credential set or removed (the value is NEVER recorded)
+    DOWNLOAD_CREDENTIAL_SET = "download_credential_set"
+    DOWNLOAD_CREDENTIAL_DELETE = "download_credential_delete"
     # The log itself was cleared (a single row survives the clear as tamper-evidence)
     AUDIT_CLEAR = "audit_clear"