Set logging levels.

There is no separate channel parameter: the {level} path segment carries both the level and the optional channel targeting. A bare level name - one of error, warn, info, debug, or excess - sets that level globally for every log channel. To target specific channels instead, use level:channel,channel and separate multiple groups with a semicolon, e.g. debug:Schedule,Player;info:Sync. Channel names are case-sensitive and match those returned by GET /api/fppd/log (for example Command, Control, HTTP, Schedule, Sync).