refactor: extract the debounced async save

YamlPlayerSettingsStorage and ConversionCache each carried a line-for-line
identical AtomicBoolean-plus-runDelayed debounce, five-second constant
included, so changing the save cadence meant changing it twice and
noticing that it was written twice.

Extracting it also removed the only reason those two classes held a
JavaPlugin: they took the whole plugin to reach the scheduler. They now
take the collaborator they actually use, which is both narrower and
testable without a running server.

ChannelStorage is deliberately left alone - it saves through runNow with
no debounce at all, and giving it one would change when writes happen.

Co-Authored-By: Claude <noreply@anthropic.com>

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