Move the option into the nested Transcoding config group alongside the
limit knobs it interacts with, so all transcoding-related settings live
together.
The old top-level name is still honored via the existing
mapDeprecatedOption / logDeprecatedOptions plumbing, which forwards the
value to the new key and logs a deprecation warning at startup. The old
struct field is removed (the new field is the single source of truth);
the deprecated default is removed so viper.IsSet correctly distinguishes
"user set the legacy option" from "no one set it."
823d851b75