Improve API payloads and graceful shutdown handling

[Sukikui]

• Remove the redundant appId field from all app event payloads
• Simplify /expired payloads to only include requestId and code
• Remove the redundant expiredAt field from request lifecycle data
• Report unfinished requests before a graceful server shutdown
• Preserve completed validations while expiring requests still waiting for a player
• Send shutdown reports concurrently with a five-second timeout
• Log shutdown progress, delivery counts, duration, and individual failures
• Clear the in-memory request store after shutdown processing
• Update the README and app integration documentation
• Add tests for shutdown request preparation