feat(T1.8): implement health checks for Kubernetes deployment

- Add internal/health package with health checker
- Implement three endpoints: /health, /health/live, /health/ready
- /health returns full JSON report with component status, latency, timestamp
- /health/live for K8s liveness probe (service running)
- /health/ready for K8s readiness probe (ready to accept traffic)
- Temporal connectivity check via GetWorkflow call with timeout
- Health check caching (30s interval) to prevent excessive checks
- Graceful shutdown: health server stops on SIGINT/SIGTERM
- Add --health flag to starter command to run health check
- Worker runs health server on port 8081 alongside task queue worker
- 10/10 unit tests passing
- All verification criteria met

Closes T1.8

internal/health/health_test.go

@@ -0,0 +1,110 @@
+package health
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// TestNewChecker tests the creation of a new health checker
+func TestNewChecker(t *testing.T) {
+	checker := NewChecker(nil)
+	assert.NotNil(t, checker)
+	assert.Equal(t, 30*time.Second, checker.checkInterval)
+}
+
+// TestCheckWithNilClient tests health check with nil client
+func TestCheckWithNilClient(t *testing.T) {
+	checker := NewChecker(nil)
+	report := checker.Check(context.Background())
+
+	assert.NotNil(t, report)
+	assert.Equal(t, StatusUnhealthy, report.Status)
+	assert.Len(t, report.Components, 1)
+	assert.Equal(t, StatusUnhealthy, report.Components["temporal"].Status)
+	assert.Equal(t, "Temporal client not initialized", report.Components["temporal"].Error)
+}
+
+// TestIsHealthy tests the IsHealthy method
+func TestIsHealthy(t *testing.T) {
+	checker := NewChecker(nil)
+	assert.False(t, checker.IsHealthy(context.Background()))
+}
+
+// TestGetReport tests the GetReport method
+func TestGetReport(t *testing.T) {
+	checker := NewChecker(nil)
+	report := checker.GetReport(context.Background())
+	assert.NotNil(t, report)
+	assert.Equal(t, StatusUnhealthy, report.Status)
+}
+
+// TestHealthReportJSON tests JSON serialization
+func TestHealthReportJSON(t *testing.T) {
+	checker := NewChecker(nil)
+	report := checker.Check(context.Background())
+
+	jsonData, err := report.ToJSON()
+	assert.NoError(t, err)
+	assert.NotEmpty(t, jsonData)
+	assert.Contains(t, string(jsonData), "unhealthy")
+	assert.Contains(t, string(jsonData), "temporal")
+}
+
+// TestHealthReportTimestamp tests that timestamp is set
+func TestHealthReportTimestamp(t *testing.T) {
+	checker := NewChecker(nil)
+	before := time.Now()
+	report := checker.Check(context.Background())
+	after := time.Now()
+
+	assert.True(t, report.Timestamp.After(before) || report.Timestamp.Equal(before))
+	assert.True(t, report.Timestamp.Before(after) || report.Timestamp.Equal(after))
+}
+
+// TestStatusString tests Status string representation
+func TestStatusString(t *testing.T) {
+	assert.Equal(t, "healthy", StatusHealthy.String())
+	assert.Equal(t, "unhealthy", StatusUnhealthy.String())
+	assert.Equal(t, "unknown", StatusUnknown.String())
+}
+
+// TestComponentHealthLatency tests that latency is recorded
+func TestComponentHealthLatency(t *testing.T) {
+	checker := NewChecker(nil)
+	report := checker.Check(context.Background())
+
+	assert.NotNil(t, report.Components["temporal"])
+	assert.GreaterOrEqual(t, report.Components["temporal"].Latency, int64(0))
+}
+
+// TestHealthCheckCaching tests that recent checks are cached
+func TestHealthCheckCaching(t *testing.T) {
+	checker := NewChecker(nil)
+
+	// First check
+	report1 := checker.Check(context.Background())
+	time1 := report1.Timestamp
+
+	// Second check immediately (should be cached)
+	time.Sleep(100 * time.Millisecond)
+	report2 := checker.Check(context.Background())
+	time2 := report2.Timestamp
+
+	// Timestamps should be the same or very close (cached)
+	assert.Equal(t, time1, time2, "second check should use cached result")
+}
+
+// TestComponentHealthDefaults tests default component health values
+func TestComponentHealthDefaults(t *testing.T) {
+	comp := ComponentHealth{
+		Name:   "test",
+		Status: StatusHealthy,
+	}
+
+	assert.Equal(t, "test", comp.Name)
+	assert.Equal(t, StatusHealthy, comp.Status)
+	assert.Empty(t, comp.Error)
+}