refactor(scheduler): extract cron scheduling logic to shared helper

Extract the shouldRunNow() method from ScheduledJobManager and ServerManagerJob into
a reusable shouldRunCronNow() helper function. This centralizes cron scheduling logic
and enables consistent deduplication behavior across all scheduled job types.

- Create shouldRunCronNow() helper in bootstrap/helpers/shared.php with timezone
  and dedup support
- Refactor ScheduledJobManager and ServerManagerJob to use the shared helper
- Add ScheduledJobDiagnostics command for inspecting cache state and scheduling
  decisions across all scheduled jobs
- Simplify shouldRunNow tests to directly test the helper function
- Add DockerCleanupJob test for error handling and execution tracking
- Increase scheduled log retention from 1 to 7 days

config/logging.php

@@ -123,7 +123,7 @@ return [
             'driver' => 'daily',
             'path' => storage_path('logs/scheduled.log'),
             'level' => 'debug',
-            'days' => 1,
+            'days' => 7,
         ],
 
         'scheduled-errors' => [