fix(backup): keep GUI backup paths under BACKUP_ROOT and limit the schedule

backup:configure could point automatic backups at any writable directory
(e.g. a statically served one) and set a cron of every second; an invalid
expression deleted the running job before failing, silently ending
backups.

- A path entered in the UI is resolved under BACKUP_ROOT (default
  ./backups) and refused if it leaves it; env values stay unrestricted.
- The schedule must be a five-field cron that runs at most hourly.
- reload() validates before replacing the running job.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

ApiCore-Frontend/src/app/(app)/settings/backup/page.tsx

@@ -225,8 +225,13 @@ function ScheduleSettingsForm({
           value={storagePath}
           disabled={envOverrides.storagePath}
           onChange={(event) => setStoragePath(event.target.value)}
-          placeholder="/data/backups"
+          placeholder="nightly"
         />
+        {!envOverrides.storagePath ? (
+          <p className="text-xs text-muted-foreground">
+            Unterordner im Backup-Verzeichnis des Servers (BACKUP_ROOT), z. B. „nightly“.
+          </p>
+        ) : null}
         {envOverrides.storagePath ? (
           <p className="text-xs text-muted-foreground">
             Durch BACKUP_STORAGE_PATH vorgegeben.
@@ -241,7 +246,11 @@ function ScheduleSettingsForm({
           disabled={envOverrides.cronSchedule}
           onChange={(event) => setCronSchedule(event.target.value)}
           placeholder="0 3 * * *"
+          aria-describedby="backup-cron-hint"
         />
+        <p id="backup-cron-hint" className="text-xs text-muted-foreground">
+          Fünf Felder, höchstens einmal pro Stunde. „0 3 * * *“ = täglich um 3 Uhr.
+        </p>
         {envOverrides.cronSchedule ? (
           <p className="text-xs text-muted-foreground">
             Durch BACKUP_CRON_SCHEDULE vorgegeben.