refactor(models): typed Status constants with IsBroken() predicate

Replace ~150 bare status string comparisons with typed models.Status
constants (StatusUp, StatusDown, StatusPending, StatusLate, StatusStale,
StatusSSLExp). Single IsBroken() method replaces the duplicated
isBroken lambda in monitor.go and isDown function in sla.go.

Adding a new status value (e.g. DEGRADED) now requires one constant
definition instead of grep-and-pray across 16 files.

CheckResult.Status stays string — the checker is the boundary between
raw protocol results and typed status. Cast happens at the edge in
handleStatusChange.

internal/models/status.go

@@ -0,0 +1,18 @@
+package models
+
+type Status string
+
+const (
+	StatusUp      Status = "UP"
+	StatusDown    Status = "DOWN"
+	StatusPending Status = "PENDING"
+	StatusLate    Status = "LATE"
+	StatusStale   Status = "STALE"
+	StatusSSLExp  Status = "SSL EXP"
+)
+
+func (s Status) IsBroken() bool {
+	return s == StatusDown || s == StatusSSLExp
+}
+
+func (s Status) String() string { return string(s) }