✨ 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>

features/health/health.feature

@@ -7,4 +7,12 @@ Feature: Health Endpoint
   Scenario: Health check returns healthy status
     Given the server is running
     When I request the health endpoint
-    Then the response should be "{\"status\":\"healthy\"}"
+    Then the response should be "{\"status\":\"healthy\"}"
+
+  @basic @critical
+  Scenario: Healthz endpoint returns rich health info
+    Given the server is running
+    When I request the healthz endpoint
+    Then the response status code should be 200
+    And the response should be JSON with fields "status, version, uptime_seconds, timestamp"
+    And the "status" field should equal "healthy"