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

pnpm-lock.yaml

@@ -413,6 +413,9 @@ importers:
       class-validator:
         specifier: ^0.15.1
         version: 0.15.1
+      cron:
+        specifier: ^4.4.0
+        version: 4.4.0
       date-fns-tz:
         specifier: ^3.2.0
         version: 3.2.0(date-fns@4.1.0)