✨ feat(server): add /api/healthz endpoint with rich health info

Adds a Kubernetes-style healthz endpoint returning status, version,
uptime_seconds and timestamp. Non-breaking — /api/health is preserved.

- New route: GET /api/healthz
- New handler: handleHealthz with HealthzResponse struct
- New unit test: pkg/server/healthz_test.go (passes locally)
- New BDD scenario: features/health/health.feature
- BDD steps: pkg/bdd/steps/health_steps.go, common_steps.go

Note: BDD tests require Postgres and will be validated by CI.

🤖 Co-Authored-By: Mistral Vibe (devstral-2 / mistral-medium-3.5)
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pkg/bdd/steps/common_steps.go

@@ -63,3 +63,39 @@ func (s *CommonSteps) theStatusCodeShouldBe(expectedStatus int) error {
 	}
 	return nil
 }
+
+// JSON field validation
+func (s *CommonSteps) theResponseShouldBeJSONWithFields(fields string) error {
+	// Parse the fields comma-separated list
+	fieldList := strings.Split(fields, ", ")
+	for _, field := range fieldList {
+		field = strings.TrimSpace(field)
+		if !s.responseContainsJSONField(field) {
+			return fmt.Errorf("response does not contain field %q", field)
+		}
+	}
+	return nil
+}
+
+func (s *CommonSteps) responseContainsJSONField(field string) bool {
+	body := string(s.client.GetLastBody())
+	// Simple check - look for "field":" in the JSON
+	// This works for simple fields, may need enhancement for nested objects
+	searchString := `"` + field + `":`
+	return strings.Contains(body, searchString)
+}
+
+func (s *CommonSteps) theFieldShouldEqual(field, expectedValue string) error {
+	body := string(s.client.GetLastBody())
+	// Look for the field and extract its value
+	// Simple implementation: look for "field":"value" pattern
+	searchPattern := `"` + field + `":"` + expectedValue + `"`
+	if !strings.Contains(body, searchPattern) {
+		// Also try without quotes (for numbers)
+		searchPatternNum := `"` + field + `":` + expectedValue
+		if !strings.Contains(body, searchPatternNum) {
+			return fmt.Errorf("field %q does not equal %q in response: %s", field, expectedValue, body)
+		}
+	}
+	return nil
+}