feat(M48): continuous TLS health monitoring — endpoint state machine, shared tlsprobe, 8 API endpoints, GUI

Adds continuous TLS endpoint health monitoring that closes the deploy→verify→monitor loop.
After M25 verifies a deployment succeeded once, M48 continuously confirms it stays healthy.

Key components:
- Shared `internal/tlsprobe/` package extracted from network scanner for reuse
- Health status state machine: healthy → degraded (2 failures) → down (5 failures),
  plus cert_mismatch when served fingerprint differs from expected
- 8th scheduler loop (60s tick, per-endpoint configurable intervals)
- PostgreSQL migration 000011: endpoint_health_checks + endpoint_health_history tables
- 8 REST API endpoints (CRUD, history, acknowledge, summary)
- Health Monitor GUI page with summary bar, status table, create modal, auto-refresh
- 38 new tests (5 tlsprobe + 11 domain + 10 service + 8 handler + 4 frontend)
- All coverage thresholds maintained (service 68%, handler 83%, domain 87%, middleware 63%)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/domain/health_check_test.go

@@ -0,0 +1,237 @@
+package domain
+
+import (
+	"testing"
+	"time"
+)
+
+func TestIsValidHealthStatus(t *testing.T) {
+	tests := []struct {
+		status string
+		valid  bool
+	}{
+		{"healthy", true},
+		{"degraded", true},
+		{"down", true},
+		{"cert_mismatch", true},
+		{"unknown", true},
+		{"invalid", false},
+		{"", false},
+		{"HEALTHY", false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.status, func(t *testing.T) {
+			result := IsValidHealthStatus(tt.status)
+			if result != tt.valid {
+				t.Errorf("IsValidHealthStatus(%q) = %v, want %v", tt.status, result, tt.valid)
+			}
+		})
+	}
+}
+
+func TestTransitionStatus_HealthyProbe(t *testing.T) {
+	h := &EndpointHealthCheck{
+		Status:              HealthStatusUnknown,
+		ConsecutiveFailures: 0,
+		DegradedThreshold:   2,
+		DownThreshold:       5,
+		ExpectedFingerprint: "abc123",
+	}
+
+	newStatus, transitioned := h.TransitionStatus(true, "abc123")
+
+	if newStatus != HealthStatusHealthy {
+		t.Errorf("expected HealthStatusHealthy, got %s", newStatus)
+	}
+	if !transitioned {
+		t.Errorf("expected transition=true, got false")
+	}
+}
+
+func TestTransitionStatus_CertMismatch(t *testing.T) {
+	h := &EndpointHealthCheck{
+		Status:              HealthStatusHealthy,
+		ConsecutiveFailures: 0,
+		DegradedThreshold:   2,
+		DownThreshold:       5,
+		ExpectedFingerprint: "abc123",
+	}
+
+	newStatus, transitioned := h.TransitionStatus(true, "xyz789")
+
+	if newStatus != HealthStatusCertMismatch {
+		t.Errorf("expected HealthStatusCertMismatch, got %s", newStatus)
+	}
+	if !transitioned {
+		t.Errorf("expected transition=true, got false")
+	}
+}
+
+func TestTransitionStatus_FirstFailure_BelowThreshold(t *testing.T) {
+	h := &EndpointHealthCheck{
+		Status:              HealthStatusHealthy,
+		ConsecutiveFailures: 0,
+		DegradedThreshold:   2,
+		DownThreshold:       5,
+	}
+
+	newStatus, transitioned := h.TransitionStatus(false, "")
+
+	// At 1 failure with degraded threshold 2, still healthy
+	if newStatus != HealthStatusHealthy {
+		t.Errorf("expected HealthStatusHealthy (grace period), got %s", newStatus)
+	}
+	if transitioned {
+		t.Errorf("expected transition=false (still healthy), got true")
+	}
+}
+
+func TestTransitionStatus_DegradedThreshold(t *testing.T) {
+	h := &EndpointHealthCheck{
+		Status:              HealthStatusHealthy,
+		ConsecutiveFailures: 1, // Now will be 2 after increment
+		DegradedThreshold:   2,
+		DownThreshold:       5,
+	}
+
+	newStatus, transitioned := h.TransitionStatus(false, "")
+
+	if newStatus != HealthStatusDegraded {
+		t.Errorf("expected HealthStatusDegraded, got %s", newStatus)
+	}
+	if !transitioned {
+		t.Errorf("expected transition=true, got false")
+	}
+}
+
+func TestTransitionStatus_DownThreshold(t *testing.T) {
+	h := &EndpointHealthCheck{
+		Status:              HealthStatusDegraded,
+		ConsecutiveFailures: 4, // Now will be 5 after increment
+		DegradedThreshold:   2,
+		DownThreshold:       5,
+	}
+
+	newStatus, transitioned := h.TransitionStatus(false, "")
+
+	if newStatus != HealthStatusDown {
+		t.Errorf("expected HealthStatusDown, got %s", newStatus)
+	}
+	if !transitioned {
+		t.Errorf("expected transition=true, got false")
+	}
+}
+
+func TestTransitionStatus_Recovery(t *testing.T) {
+	h := &EndpointHealthCheck{
+		Status:              HealthStatusDown,
+		ConsecutiveFailures: 10,
+		DegradedThreshold:   2,
+		DownThreshold:       5,
+		ExpectedFingerprint: "abc123",
+	}
+
+	newStatus, transitioned := h.TransitionStatus(true, "abc123")
+
+	if newStatus != HealthStatusHealthy {
+		t.Errorf("expected HealthStatusHealthy (recovery), got %s", newStatus)
+	}
+	if !transitioned {
+		t.Errorf("expected transition=true (from down to healthy), got false")
+	}
+}
+
+func TestTransitionStatus_NoFingerprint(t *testing.T) {
+	h := &EndpointHealthCheck{
+		Status:              HealthStatusHealthy,
+		ConsecutiveFailures: 0,
+		DegradedThreshold:   2,
+		DownThreshold:       5,
+		ExpectedFingerprint: "", // No expected fingerprint
+	}
+
+	newStatus, transitioned := h.TransitionStatus(true, "anything")
+
+	// Success with no expected fingerprint should always be healthy
+	if newStatus != HealthStatusHealthy {
+		t.Errorf("expected HealthStatusHealthy (no fingerprint check), got %s", newStatus)
+	}
+	if transitioned {
+		t.Errorf("expected transition=false (already healthy), got true")
+	}
+}
+
+func TestTransitionStatus_UnknownToHealthy(t *testing.T) {
+	h := &EndpointHealthCheck{
+		Status:              HealthStatusUnknown,
+		ConsecutiveFailures: 0,
+		DegradedThreshold:   2,
+		DownThreshold:       5,
+	}
+
+	newStatus, transitioned := h.TransitionStatus(true, "")
+
+	if newStatus != HealthStatusHealthy {
+		t.Errorf("expected HealthStatusHealthy, got %s", newStatus)
+	}
+	if !transitioned {
+		t.Errorf("expected transition=true (from unknown to healthy), got false")
+	}
+}
+
+func TestTransitionStatus_NoTransitionWhenSame(t *testing.T) {
+	h := &EndpointHealthCheck{
+		Status:              HealthStatusHealthy,
+		ConsecutiveFailures: 0,
+		DegradedThreshold:   2,
+		DownThreshold:       5,
+	}
+
+	newStatus, transitioned := h.TransitionStatus(true, "")
+
+	if newStatus != HealthStatusHealthy {
+		t.Errorf("expected HealthStatusHealthy, got %s", newStatus)
+	}
+	if transitioned {
+		t.Errorf("expected transition=false (already healthy), got true")
+	}
+}
+
+func TestHealthCheckSummary(t *testing.T) {
+	summary := &HealthCheckSummary{
+		Healthy:      5,
+		Degraded:     2,
+		Down:         1,
+		CertMismatch: 1,
+		Unknown:      0,
+		Total:        9,
+	}
+
+	if summary.Total != 9 {
+		t.Errorf("expected Total=9, got %d", summary.Total)
+	}
+	if summary.Healthy != 5 {
+		t.Errorf("expected Healthy=5, got %d", summary.Healthy)
+	}
+}
+
+func TestHealthHistoryEntry(t *testing.T) {
+	now := time.Now()
+	entry := &HealthHistoryEntry{
+		ID:             "hh-test-123",
+		HealthCheckID:  "hc-test-123",
+		Status:         "healthy",
+		ResponseTimeMs: 42,
+		Fingerprint:    "abc123def456",
+		FailureReason:  "",
+		CheckedAt:      now,
+	}
+
+	if entry.ID != "hh-test-123" {
+		t.Errorf("expected ID='hh-test-123', got %q", entry.ID)
+	}
+	if entry.ResponseTimeMs != 42 {
+		t.Errorf("expected ResponseTimeMs=42, got %d", entry.ResponseTimeMs)
+	}
+}