Add backpressure to ListenerExecutor task submission

[sfloess]

Part of #423: Race condition potential in ApplicationManager concurrent operations

Problem

The ListenerExecutor uses unbounded task submission. Under heavy load (e.g., rapid deploy/undeploy cycles), this can exhaust memory or cause listener notifications to queue unboundedly, leading to OOM or extreme latency for listener callbacks.

Location

• platform-core/src/main/java/org/flossware/platform/core/ApplicationManager.java (ListenerExecutor usage)

Tasks

• Identify all ListenerExecutor submission points
• Add a bounded queue or semaphore-based backpressure mechanism
• Define a rejection policy (caller-runs, discard-oldest, or block)
• Add monitoring/logging when backpressure is applied
• Ensure listener notification order is preserved where required

Acceptance Criteria

• ListenerExecutor cannot accumulate unbounded pending tasks
• Backpressure policy is documented and configurable
• No listener notifications are silently dropped without logging