Add comprehensive instance-level diagnostics to /api/monitoring/scheduler/health
**New Response Structure:**
Enhanced "instances" array with per-instance details:
- Instance metadata: displayName, type, connection URL
- Poll status: last success/error timestamps, error messages, error category
- Circuit breaker: state, timestamps, failure counts, retry windows
- Dead letter: present flag, reason, attempt history, retry schedule
**Implementation:**
Data structures:
- instanceInfo: cache of display names, URLs, types
- pollStatus: tracks successes/errors with timestamps and categories
- dlqInsight: DLQ entry metadata (reason, attempts, schedule)
- circuitBreaker: enhanced with stateSince, lastTransition
Tracking logic:
- buildInstanceInfoCache: populate metadata from config on startup
- recordTaskResult: track poll outcomes, error details, categories
- sendToDeadLetter: capture DLQ insights (reason, timestamps)
- circuitBreaker: record state transitions with timestamps
**Backward Compatible:**
- Existing fields (deadLetter, breakers, staleness) unchanged
- New "instances" array is additive
- Old clients can ignore new fields
**Testing:**
- Unit test: TestSchedulerHealth_EnhancedResponse validates all fields
- Integration tests: still passing (55s)
- All error tracking and breaker history verified
**Operator Benefits:**
- Diagnose issues without log digging
- See error messages directly in API
- Understand breaker states and retry schedules
- Track DLQ entries with full context
- Single API call for complete instance health view
Example: Quickly identify "401 unauthorized" on specific PBS instance,
see it's in DLQ after 5 retries, and know when next retry scheduled.
Part of Phase 2 follow-up work to improve observability.
