* feat(scheduler): add CrontabSchedule with crontab(5) random ~ syntax
Implement ParseCrontab() that extends robfig/cron with support for
the crontab(5) random ~ operator (e.g., 0~30 * * * *). Random values
are resolved fresh on each Next() call for load spreading.
Supports A~B, ~B, A~, and bare ~ forms in all 6 fields (including
seconds). Expressions without ~ delegate to robfig's standard parser
with zero overhead.
Integrates into scheduler.Add() and conf.validateSchedule() so that
scanner.schedule and backup.schedule config values accept ~ syntax.
* refactor(scheduler): resolve random ~ values once at parse time
Change from per-Next() randomization to per-parse randomization,
matching crontab(5) semantics. This prevents double-firing within
the same period when random values land after the current time.
ParseCrontab now resolves ~ fields to concrete values, substitutes
them into the spec string, and delegates to robfig's parser. This
eliminates CrontabSchedule, randomField, and resolveField entirely.
* test(scheduler): replace WaitGroup with channel for job execution synchronization
Signed-off-by: Deluan <deluan@navidrome.org>
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Signed-off-by: Deluan <deluan@navidrome.org>
Deluan Quintão