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.go

@@ -0,0 +1,109 @@
+package domain
+
+import "time"
+
+// HealthStatus represents the current health state of a monitored endpoint.
+type HealthStatus string
+
+const (
+	HealthStatusHealthy      HealthStatus = "healthy"
+	HealthStatusDegraded     HealthStatus = "degraded"
+	HealthStatusDown         HealthStatus = "down"
+	HealthStatusCertMismatch HealthStatus = "cert_mismatch"
+	HealthStatusUnknown      HealthStatus = "unknown"
+)
+
+// IsValidHealthStatus checks if a health status string is valid.
+func IsValidHealthStatus(s string) bool {
+	switch HealthStatus(s) {
+	case HealthStatusHealthy, HealthStatusDegraded, HealthStatusDown, HealthStatusCertMismatch, HealthStatusUnknown:
+		return true
+	}
+	return false
+}
+
+// EndpointHealthCheck represents a monitored TLS endpoint.
+type EndpointHealthCheck struct {
+	ID                  string        `json:"id"`
+	Endpoint            string        `json:"endpoint"`
+	CertificateID       *string       `json:"certificate_id,omitempty"`
+	NetworkScanTargetID *string       `json:"network_scan_target_id,omitempty"`
+	ExpectedFingerprint string        `json:"expected_fingerprint"`
+	ObservedFingerprint string        `json:"observed_fingerprint"`
+	Status              HealthStatus  `json:"status"`
+	ConsecutiveFailures int           `json:"consecutive_failures"`
+	ResponseTimeMs      int           `json:"response_time_ms"`
+	TLSVersion          string        `json:"tls_version"`
+	CipherSuite         string        `json:"cipher_suite"`
+	CertSubject         string        `json:"cert_subject"`
+	CertIssuer          string        `json:"cert_issuer"`
+	CertExpiry          *time.Time    `json:"cert_expiry,omitempty"`
+	LastCheckedAt       *time.Time    `json:"last_checked_at,omitempty"`
+	LastSuccessAt       *time.Time    `json:"last_success_at,omitempty"`
+	LastFailureAt       *time.Time    `json:"last_failure_at,omitempty"`
+	LastTransitionAt    *time.Time    `json:"last_transition_at,omitempty"`
+	FailureReason       string        `json:"failure_reason"`
+	DegradedThreshold   int           `json:"degraded_threshold"`
+	DownThreshold       int           `json:"down_threshold"`
+	CheckIntervalSecs   int           `json:"check_interval_seconds"`
+	Enabled             bool          `json:"enabled"`
+	Acknowledged        bool          `json:"acknowledged"`
+	AcknowledgedBy      string        `json:"acknowledged_by,omitempty"`
+	AcknowledgedAt      *time.Time    `json:"acknowledged_at,omitempty"`
+	CreatedAt           time.Time     `json:"created_at"`
+	UpdatedAt           time.Time     `json:"updated_at"`
+}
+
+// TransitionStatus computes the new health status based on the probe result.
+// Returns the new status and whether a transition occurred.
+func (h *EndpointHealthCheck) TransitionStatus(probeSuccess bool, observedFingerprint string) (HealthStatus, bool) {
+	oldStatus := h.Status
+	var newStatus HealthStatus
+
+	if probeSuccess {
+		if h.ExpectedFingerprint != "" && observedFingerprint != h.ExpectedFingerprint {
+			newStatus = HealthStatusCertMismatch
+		} else {
+			newStatus = HealthStatusHealthy
+		}
+	} else {
+		// Increment failures for next calculation (caller will update h.ConsecutiveFailures)
+		failures := h.ConsecutiveFailures + 1
+		if failures >= h.DownThreshold {
+			newStatus = HealthStatusDown
+		} else if failures >= h.DegradedThreshold {
+			newStatus = HealthStatusDegraded
+		} else {
+			// Keep current status during initial failures before threshold
+			// Unless we were in an error state, transition to degraded after first failure
+			if h.Status == HealthStatusUnknown || h.Status == HealthStatusHealthy {
+				newStatus = HealthStatusHealthy // still considered healthy during grace period
+			} else {
+				newStatus = h.Status
+			}
+		}
+	}
+
+	return newStatus, newStatus != oldStatus
+}
+
+// HealthHistoryEntry represents a single probe record.
+type HealthHistoryEntry struct {
+	ID            string    `json:"id"`
+	HealthCheckID string    `json:"health_check_id"`
+	Status        string    `json:"status"`
+	ResponseTimeMs int      `json:"response_time_ms"`
+	Fingerprint   string    `json:"fingerprint"`
+	FailureReason string    `json:"failure_reason"`
+	CheckedAt     time.Time `json:"checked_at"`
+}
+
+// HealthCheckSummary contains aggregate counts by status.
+type HealthCheckSummary struct {
+	Healthy      int `json:"healthy"`
+	Degraded     int `json:"degraded"`
+	Down         int `json:"down"`
+	CertMismatch int `json:"cert_mismatch"`
+	Unknown      int `json:"unknown"`
+	Total        int `json:"total"`
+}