CreateChannelFromLineupHandler resolved every advanced override with
advanced.X ?? template.X, so null always meant INHERIT and a channel could
not drop a template-set watermark / filler / preferred language. Add an
optional typed `clear` enum list to CreateChannelFromLineupAdvancedOptions:
omitted/null still inherits (byte-stable for existing clients), a field named
in `clear` is forced to none. Set+clear of the same field is a 422.
The enum (CreateChannelFromLineupClearField) lives in ErsatzTV.Core so the
OpenAPI string-enum scan renders it as a string enum, matching every sibling
advanced-options enum. Handler resolves clearable fields once via
ResolveClearable and validates set/clear conflicts via ValidateClear;
reference validation skips existence checks for cleared (null) refs.
SPA: the shared advancedOptions model re-adds a real "None" option to the five
id selects (watermark + fillers) in both the Channel Builder and the Auto-Tune
DetailPanel, routed through a CLEAR overrides sentinel that applyOverridesToRequest
folds into advanced.clear (never leaking onto the wire as a field value). The
backend enum also covers the preferred audio/subtitle language strings for
machine clients; the SPA text inputs keep "empty = inherit" (tri-state deferred).
Docs: api-conventions.md §2, spa-conventions.md §11, decisions.md record
api.from-lineup-clear-to-none; v1.json + generated TS regenerated.
fixes#135
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>