Synchronize Health Endpoint Reads of Scheduler State

[henry0816191]

Problem

The eval (Test 13, concurrency-boundaries cluster) found that the health endpoint reads scheduler state without synchronization. The scheduler runs in the main asyncio event loop (or a background thread via asyncio.to_thread), while the health endpoint serves HTTP requests that read the scheduler's last-run timestamp, cycle count, and status fields. These reads are not synchronized — the health endpoint can observe a partially-updated scheduler state (e.g., cycle count incremented but status not yet updated). This is part of the Silent-Staleness Cascade compound: the operator's primary observability signal cannot be trusted.

Acceptance Criteria

• Shared state between scheduler and health endpoint protected by a synchronization primitive (asyncio.Lock, threading.Lock, or atomic snapshot pattern)
• Health endpoint reads a consistent snapshot of scheduler state (all fields from the same point in time)
• Scheduler updates state atomically — all fields updated under the same lock acquisition
• Health endpoint response includes a last_updated timestamp indicating when the snapshot was taken
• Tests verify: concurrent scheduler update and health endpoint read produce consistent state
• Tests pass in CI (if applicable)
• PR approved by at least 1 reviewer

Implementation Notes

The cleanest approach is an atomic snapshot pattern: the scheduler writes to a SchedulerSnapshot dataclass (frozen, immutable) and publishes it to an asyncio.Event-guarded shared reference. The health endpoint reads the latest snapshot — since the snapshot object is immutable and Python object assignment is atomic (due to the GIL), a simple self._latest_snapshot = new_snapshot suffices for thread safety. If asyncio.to_thread is used for blocking I/O (the eval notes this as an extension precedent), ensure the lock discipline covers the cross-thread boundary. The eval finding on _Entry positional tuple fragility is a related but separate concern.

References

• Eval finding: Test 13 (health endpoint reads scheduler state without synchronization)
• Related files: Health endpoint handler (likely in src/api.py or src/health.py), scheduler class (likely in src/scheduler.py), shared state module