[Console] Reduce watcher poll frequency and deduplicate in-flight polls

[shangeyao]

Problem

HTTP watchers wake every 1 second even when no poll is due, and enqueue overlapping REST tasks per application when responses are slow.

Proposed solution

• Poll Flink/Spark apps every 5 seconds in steady state; keep 1-second fast polling only during start/stop operations
• Align Flink cluster watcher schedule with its 30-second interval
• Skip overlapping watch tasks per application with single-flight guards

[shangeyao]

Closing because related PR #4397 was closed without merge. Console frontend is being refactored; this work is deferred.