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>

conf/configuration.go

@@ -16,8 +16,8 @@ import (
 	"github.com/kr/pretty"
 	"github.com/navidrome/navidrome/consts"
 	"github.com/navidrome/navidrome/log"
+	"github.com/navidrome/navidrome/scheduler"
 	"github.com/navidrome/navidrome/utils/run"
-	"github.com/robfig/cron/v3"
 	"github.com/spf13/viper"
 )
 
@@ -570,15 +570,9 @@ func validateBackupSchedule() error {
 }
 
 func validateSchedule(schedule, field string) (string, error) {
-	if _, err := time.ParseDuration(schedule); err == nil {
-		schedule = "@every " + schedule
-	}
-	c := cron.New()
-	id, err := c.AddFunc(schedule, func() {})
+	_, err := scheduler.ParseCrontab(schedule)
 	if err != nil {
 		log.Error(fmt.Sprintf("Invalid %s. Please read format spec at https://pkg.go.dev/github.com/robfig/cron#hdr-CRON_Expression_Format", field), "schedule", schedule, err)
-	} else {
-		c.Remove(id)
 	}
 	return schedule, err
 }