feat: show error reason when monitors go DOWN

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.

internal/store/postgres.go

@@ -72,6 +72,15 @@ func (d *PostgresDialect) CreateTablesSQL() []string {
 			key TEXT PRIMARY KEY,
 			value TEXT NOT NULL
 		)`,
+		`CREATE TABLE IF NOT EXISTS state_changes (
+			id SERIAL PRIMARY KEY,
+			site_id INTEGER NOT NULL,
+			from_status TEXT NOT NULL,
+			to_status TEXT NOT NULL,
+			error_reason TEXT DEFAULT '',
+			changed_at TIMESTAMP DEFAULT NOW()
+		)`,
+		`CREATE INDEX IF NOT EXISTS idx_state_changes_site ON state_changes(site_id, changed_at DESC)`,
 	}
 }