* fix(plugins): discard buffered scrobbles when a plugin is removed
Scrobbles are buffered in the DB per service, keyed by the plugin name.
When a plugin was removed (deleted from the plugins folder and detected by
the sync), its pending buffer entries were left behind forever: the drain
goroutine is stopped on the next scrobbler refresh, so the rows were never
retried nor discarded. Worse, if a plugin with the same name was installed
later, the stale entries would be drained into it - potentially a completely
unrelated plugin that just reuses the name.
Add a Discard(service) method to ScrobbleBufferRepository and call it from
removePluginFromDB, right after the plugin record is deleted. Disabling a
plugin intentionally keeps its buffered scrobbles, consistent with the
buffer's purpose of surviving temporary outages, and transient unload/reload
cycles during config updates are unaffected since they never delete the
plugin record.
* fix(plugins): don't wipe builtin scrobbler queues on plugin removal
Buffer entries are keyed by service name only, and removePluginFromDB runs
for any removed plugin file, so removing a plugin named e.g. lastfm.ndp -
regardless of its capability - would discard the builtin Last.fm retry
queue. Skip the discard when the plugin name is owned by a registered
builtin scrobbler, exposed via a new scrobbler.IsBuiltinScrobbler helper.
Reported by Codex review on the PR.
Also drop the testBroker usage from the new removePluginFromDB spec: it is
defined in manager_test.go which is excluded on Windows, breaking the
Windows test build. sendPluginRefreshEvent is nil-safe, so no broker is
needed.
