Show verified cron worker cadence (#230)

Expose responsive worker cadence proof in the Cron operations dashboard.

docs/pr-previews/fix-automatic-cron-execution/README.md

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+# Automatic cron execution visual comparison
+
+The Cron workers panel now exposes machine-verifiable scheduler cadence.
+The summary reports how many active workers have maintained the required
+once-per-minute cadence, and each worker row shows its latest loop gap and
+consecutive qualifying loops.
+
+A worker is verified only after two consecutive loops, with no gap above
+60 seconds, while the worker is running and its latest observation is no more
+than 60 seconds old.
+
+## Mobile
+
+- [Before](before-mobile.png)
+- [After](after-mobile.png)
+
+## Tablet
+
+- [Before](before-tablet.png)
+- [After](after-tablet.png)
+
+## Desktop
+
+- [Before](before-desktop.png)
+- [After](after-desktop.png)
+
+The updated state tags use explicit foreground colors so success and warning
+labels remain readable against their backgrounds.