feat(scheduler): add crontab(5) random ~ syntax support (#5233)

* 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>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

scheduler/scheduler.go

@@ -33,10 +33,11 @@ func (s *scheduler) Run(ctx context.Context) {
 }
 
 func (s *scheduler) Add(crontab string, cmd func()) (int, error) {
-	entryID, err := s.c.AddFunc(crontab, cmd)
+	schedule, err := ParseCrontab(crontab)
 	if err != nil {
 		return 0, err
 	}
+	entryID := s.c.Schedule(schedule, cron.FuncJob(cmd))
 	return int(entryID), nil
 }