Add outbox message age monitoring

[Sarmkadan]

Summary

There is no alert when outbox messages are stuck and growing old. A metric for the oldest unprocessed message would help detect processing failures.

What to do

1. In OutboxProcessor, query the oldest unprocessed message timestamp each cycle
2. Log a warning when the oldest message exceeds a configurable threshold (default 5 minutes)
3. Add OldestMessageAge to the health check data

Files to reference

• src/Processing/OutboxProcessor.cs - polling loop
• src/Health/OutboxHealthCheck.cs - health check

Simple monitoring addition.