storage + wiring + API for the channel settings modal (UI next):
- db.get/set_channel_settings(channel_id): per-channel {custom_name, quality}
in the settings KV store (no schema change); blanks clear the override.
- enqueue applies them: custom_name overrides the $channel folder token; a
quality override is stashed in the download row's search_ctx. the worker reads
it back (quality_override_from_download) and uses it instead of the global
youtube quality profile. global default still applies when no override.
- API GET/POST /youtube/channel/<id>/settings (GET also returns the global
default quality for the modal's 'using default' hint).
pure helpers (enqueue_ctx, quality_override_from_download) + db + api seam-tested.
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