✨ 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/steps.go

@@ -83,6 +83,7 @@ func InitializeAllSteps(ctx *godog.ScenarioContext, client *testserver.Client, s
 
 	// Health steps
 	ctx.Step(`^I request the health endpoint$`, sc.healthSteps.iRequestTheHealthEndpoint)
+	ctx.Step(`^I request the healthz endpoint$`, sc.healthSteps.iRequestTheHealthzEndpoint)
 	ctx.Step(`^the server is running$`, sc.healthSteps.theServerIsRunning)
 
 	// Auth steps
@@ -297,4 +298,6 @@ func InitializeAllSteps(ctx *godog.ScenarioContext, client *testserver.Client, s
 	ctx.Step(`^the response should be "{\\"([^"]*)":\\"([^"]*)"}"$`, sc.commonSteps.theResponseShouldBe)
 	ctx.Step(`^the response should contain error "([^"]*)"$`, sc.commonSteps.theResponseShouldContainError)
 	ctx.Step(`^the status code should be (\d+)$`, sc.commonSteps.theStatusCodeShouldBe)
+	ctx.Step(`^the response should be JSON with fields "([^"]*)"$`, sc.commonSteps.theResponseShouldBeJSONWithFields)
+	ctx.Step(`^the "([^"]*)" field should equal "([^"]*)"$`, sc.commonSteps.theFieldShouldEqual)
 }