feat(server): add go_version to /api/info response (#54)

Co-authored-by: Gabriel Radureau <arcodange@gmail.com>
Co-committed-by: Gabriel Radureau <arcodange@gmail.com>

adr/0026-composite-info-endpoint.md

@@ -141,10 +141,13 @@ We will implement a new `GET /api/info` endpoint that returns a JSON object with
   "build_date": "2026-05-04T08:00:00Z",
   "uptime_seconds": 1234,
   "cache_enabled": true,
-  "healthz_status": "healthy"
+  "healthz_status": "healthy",
+  "go_version": "go1.26.1"
 }
 ```
 
+The `go_version` field provides the Go runtime version via `runtime.Version()`, useful for ops debugging (e.g., identifying which Go version is running in production).
+
 ### Rationale
 
 1. **Performance**: Single HTTP request instead of 3-4 separate calls

documentation/API.md

@@ -30,7 +30,8 @@ Reference document for all HTTP endpoints exposed by `dance-lessons-coach` serve
   "build_date": "2026-05-05",
   "uptime_seconds": 1234,
   "cache_enabled": true,
-  "healthz_status": "healthy"
+  "healthz_status": "healthy",
+  "go_version": "go1.26.1"
 }
 ```
 

features/info/info.feature

@@ -36,3 +36,10 @@ Feature: Info Endpoint
     Then the response header "X-Cache" should be "MISS"
     When I request the info endpoint again
     Then the response header "X-Cache" should be "HIT"
+
+  @go_version @critical
+  Scenario: go_version field is non-empty
+    Given the server is running
+    When I request the info endpoint
+    Then the status code should be 200
+    And the response should contain "go_version"

pkg/server/server.go

@@ -9,6 +9,7 @@ import (
 	"net"
 	"net/http"
 	"os/signal"
+	"runtime"
 	"syscall"
 	"time"
 
@@ -453,6 +454,7 @@ type InfoResponse struct {
 	UptimeSeconds int64  `json:"uptime_seconds"`
 	CacheEnabled  bool   `json:"cache_enabled"`
 	HealthzStatus string `json:"healthz_status"`
+	GoVersion     string `json:"go_version"`
 }
 
 // handleHealthz godoc
@@ -500,6 +502,7 @@ func (s *Server) handleInfo(w http.ResponseWriter, r *http.Request) {
 		UptimeSeconds: int64(time.Since(s.startedAt).Seconds()),
 		CacheEnabled:  s.cacheService != nil,
 		HealthzStatus: "healthy",
+		GoVersion:     runtime.Version(),
 	}
 
 	// Cache key