fix(library): run scans on the stored auto-scan schedule

The auto-scan cron expression was validated, persisted and shown in the UI, but nothing ever
consumed it, so a library configured with a schedule never scanned and no scan job was ever
recorded with the schedule trigger.

Register a cron job per configured library on boot and keep it in step as libraries are created,
updated and deleted. An occurrence that lands while a scan is already running is skipped rather
than retried, and a failing occurrence leaves the job registered so later ones still fire.

Schedules run in the server timezone, matching the cleanup, audit and statistics jobs, so a
deployment that sets TZ gets the wall-clock time its users expect.

Move cron validation into a single shared grammar used by both the API and the schedule editor.
The two sides previously disagreed in both directions: the UI accepted step-zero and reversed
ranges that the API rejected, and rejected day names the API accepted. The grammar is a strict
subset of what the scheduler and the human-readable preview can each parse, so an expression the
editor marks valid can always be scheduled.

Closes #985

client/src/features/library/components/LibraryCreatorSchedule.vue

@@ -3,6 +3,7 @@ import { computed } from 'vue'
 import { useI18n } from 'vue-i18n'
 import { Eye } from '@lucide/vue'
 import cronstrue from 'cronstrue'
+import { isFiveFieldCronExpression } from '@bookorbit/types'
 import ToggleSwitch from '@/components/ui/ToggleSwitch.vue'
 
 const { t } = useI18n()
@@ -27,8 +28,6 @@ const presets = computed(() => [
   { label: t('library.creator.schedule.presets.custom'), value: '__custom__' },
 ])
 
-const CRON_REGEX = /^((\*|\d+(-\d+)?(,\d+(-\d+)?)*)(\/\d+)? ){4}(\*|\d+(-\d+)?(,\d+(-\d+)?)*)(\/\d+)?$/
-
 const isCustom = computed(() => {
   if (props.autoScanCronExpression === null) return false
   return !presets.value.some((p) => p.value === props.autoScanCronExpression)
@@ -36,7 +35,7 @@ const isCustom = computed(() => {
 
 const isCronValid = computed(() => {
   if (!isCustom.value || !props.autoScanCronExpression) return true
-  return CRON_REGEX.test(props.autoScanCronExpression)
+  return isFiveFieldCronExpression(props.autoScanCronExpression)
 })
 
 const selectedPreset = computed(() => {
@@ -74,7 +73,7 @@ function humanReadableCron(cron: string | null): string {
   try {
     return cronstrue.toString(cron)
   } catch {
-    return 'Enter a valid schedule to see a preview'
+    return t('library.creator.schedule.human.invalid')
   }
 }
 </script>
@@ -98,6 +97,7 @@ function humanReadableCron(cron: string | null): string {
     <!-- Auto-scan schedule -->
     <div>
       <p class="text-[11px] font-semibold uppercase tracking-widest text-foreground mb-3">{{ t('library.creator.schedule.autoScanSchedule') }}</p>
+      <p class="mb-3 text-xs text-muted-foreground">{{ t('library.creator.schedule.timezoneHint') }}</p>
       <div class="mb-4 grid grid-cols-2 gap-2 sm:grid-cols-3">
         <button
           v-for="preset in presets"

client/src/features/library/composables/useLibraryCreator.ts

@@ -1,6 +1,6 @@
 import { computed, reactive, ref } from 'vue'
 import { api } from '@/lib/api'
-import { DEFAULT_FORMAT_PRIORITY, FORMAT_LABELS } from '@bookorbit/types'
+import { DEFAULT_FORMAT_PRIORITY, FORMAT_LABELS, isFiveFieldCronExpression } from '@bookorbit/types'
 import type { CoverAspectRatio, Library, OrganizationMode, PrescanResult } from '@bookorbit/types'
 
 export { DEFAULT_FORMAT_PRIORITY, FORMAT_LABELS }
@@ -14,7 +14,6 @@ export const METADATA_LABELS: Record<string, string> = {
 
 export type LibraryCreatorSectionId = 'details' | 'folders' | 'scanner' | 'metadata' | 'reading' | 'schedule' | 'fileWrite' | 'access'
 
-const CRON_REGEX = /^((\*|\d+(-\d+)?(,\d+(-\d+)?)*)(\/\d+)? ){4}(\*|\d+(-\d+)?(,\d+(-\d+)?)*)(\/\d+)?$/
 const FILE_SIZE_MIN_MB = 1
 const FILE_SIZE_MAX_MB = 10_000
 
@@ -66,7 +65,7 @@ export function useLibraryCreator() {
     if (!form.name.trim()) errors.details = 'Enter a library name.'
     else if (!form.icon?.trim()) errors.details = 'Choose an icon.'
     if (form.folders.length === 0) errors.folders = 'Add at least one folder.'
-    if (form.autoScanCronExpression && !CRON_REGEX.test(form.autoScanCronExpression)) {
+    if (form.autoScanCronExpression && !isFiveFieldCronExpression(form.autoScanCronExpression)) {
       errors.schedule = 'Enter a valid 5-field cron expression.'
     }
     if (form.readingThreshold < 0.05 || form.readingThreshold > 5) {

client/src/features/library/utils/__tests__/cron.spec.ts

@@ -1,7 +1,25 @@
 import { describe, expect, it } from 'vitest'
+import { isFiveFieldCronExpression } from '@bookorbit/types'
 
 import { parseCronToHuman } from '../cron'
 
+describe('isFiveFieldCronExpression', () => {
+  it.each(['0 4 * * *', '*/30 * * * *', '0 0 * * 1', '0 4 * * MON', '0  4 * * *', '1-5/2 * * * *', '0 4 * jan-mar *'])(
+    'accepts %s and previews it',
+    (expression) => {
+      expect(isFiveFieldCronExpression(expression)).toBe(true)
+      expect(parseCronToHuman(expression)).not.toBe(expression)
+    },
+  )
+
+  it.each(['not a cron', '0 99 * * *', '* * * * * *', '*/0 * * * *', '5-1 * * * *', '0 4 */0 * *', '@daily', '0 4 * * FRI-MON'])(
+    'rejects %s',
+    (expression) => {
+      expect(isFiveFieldCronExpression(expression)).toBe(false)
+    },
+  )
+})
+
 describe('parseCronToHuman', () => {
   it('returns null for null input', () => {
     expect(parseCronToHuman(null)).toBeNull()

client/src/locales/en.json

@@ -5672,6 +5672,7 @@
       "schedule": {
         "fileWatching": "File watching",
         "autoScanSchedule": "Auto-scan schedule",
+        "timezoneHint": "Schedules run in the server's timezone.",
         "cronExpression": "Cron expression",
         "cronInvalid": "Invalid cron expression.",
         "cronFormat": "Format: minute hour day month weekday",
@@ -5690,6 +5691,7 @@
         },
         "human": {
           "disabled": "Auto-scan disabled",
+          "invalid": "Enter a valid schedule to see a preview",
           "hourly": "Every hour",
           "every6Hours": "Every 6 hours",
           "every12Hours": "Every 12 hours",