feat(Program): make Quartz cron schedule configurable via appsettings

- Replaced hardcoded cron expression with configuration-based `Worker:CronExpression`.
- Throws descriptive exception if cron expression is missing.
- Keeps previous worker and DB context setup unchanged.

EnvelopeGenerator.Finalizer/appsettings.Worker.json

@@ -1,5 +1,5 @@
 {
   "Worker": {
-    "IntervalInMin": 0.01666666666
+    "CronExpression": "* * * * * ?"
   }
 }