fix(downloads): a YouTube clip goes with the user's own cookie and is diagnosed like a download

A clip fetched its range signed out, so a video behind YouTube's per-video "not a bot" wall could
never be clipped while its download went through with the user's cookie - and the failure copy told
the user to add a cookie the clip would not use. The clip asset is private by construction
(`clip.asset_ref` names its user), so `usable_clip_cookie` attaches the user's YouTube cookie on that
identity, and a failed range fetch now carries the download catalog's diagnosis
(`youtube.bot_wall` / `youtube.cookie_rejected`) instead of a bare category.

backend/app/downloads/credentials.py

@@ -30,6 +30,7 @@ from sqlalchemy import select
 from sqlalchemy.orm import Session
 
 from app import logs
+from app.downloads import clip as clipmod
 from app.downloads import formats
 from app.downloads.adapters import _host_is
 from app.models import DownloadCredential, MediaAsset
@@ -259,6 +260,28 @@ def usable_cookie(
         return None
     if not is_authed_asset(asset, spec, user_id, url):
         return None  # shared asset — never attach a cookie to a non-private identity
+    return _readable(cred, user_id, host_key)
+
+
+def usable_clip_cookie(
+    db: Session, user_id: int, url: str | None, asset: MediaAsset, clip_spec: dict
+) -> str | None:
+    """`usable_cookie` for an ad-hoc clip's range fetch. The second gate is the clip's own identity:
+    a clip asset is private by construction (`clip.asset_ref` names its user, see downloads/clip),
+    so a cookie goes with it exactly when that asset is THIS user's clip of `clip_spec`. Without it a
+    video YouTube gates signed out could never be clipped, while its download goes through."""
+    host_key = host_key_for_url(url)
+    if host_key is None:
+        return None
+    cred = get_credential(db, user_id, host_key)
+    if cred is None:
+        return None
+    if asset.source_kind != "clip" or asset.source_ref != clipmod.asset_ref(user_id, clip_spec):
+        return None
+    return _readable(cred, user_id, host_key)
+
+
+def _readable(cred: DownloadCredential, user_id: int, host_key: str) -> str | None:
     raw = decrypt_optional(cred.cookies_enc)
     if not raw:
         log.warning(
@@ -284,6 +307,14 @@ def resolve_cookiefile(
     return _write_cookiefile(raw, staging) if raw else None
 
 
+def resolve_clip_cookiefile(
+    db: Session, user_id: int, url: str | None, asset: MediaAsset, clip_spec: dict, staging: Path
+) -> Path | None:
+    """`resolve_cookiefile` for an ad-hoc clip (`usable_clip_cookie`)."""
+    raw = usable_clip_cookie(db, user_id, url, asset, clip_spec)
+    return _write_cookiefile(raw, staging) if raw else None
+
+
 def _write_cookiefile(raw: str, staging: Path) -> Path:
     """Write the cookie blob to a private 0600 file under `staging/.auth/` and return its path.