Propagate check failure reasons through the entire stack:
- Checker captures specific errors (DNS, timeout, HTTP status, SSL, etc.)
- Engine tracks LastError, StatusChangedAt, LastSuccessAt per monitor
- State transitions persisted to new state_changes table
- Detail panel shows error reason, HTTP code, state duration, last
success time, and last 5 state change events
- Monitor table shows inline error preview for DOWN services
- Alert messages include error reason
- Probe nodes forward error reasons to leader
15 files changed across models, checker, engine, store, TUI, and probes.
55 tests covering state machine transitions, heartbeat handling, push
deadline checks, group aggregation, history recording, probe aggregation,
log management, state management, and concurrency safety.
Checker tests cover HTTP (via httptest), port (via net.Listen),
isCodeAccepted ranges, and siteTimeout defaults. Ping and DNS
checkers skipped (need ICMP privileges and DNS server).
Coverage: 64.2% overall, 100% on handleStatusChange, triggerAlert,
checkPush, recordCheck, and AggregateStatus.