feat(buildinfo): add build metadata and expose via HTTP headers

Introduce a new `buildinfo` package to store version, commit, and build date metadata. Update HTTP handlers to include build metadata in response headers and modify initialization to set `buildinfo` values during runtime.

internal/api/handlers/management/handler.go

@@ -13,6 +13,7 @@ import (
 	"time"
 
 	"github.com/gin-gonic/gin"
+	"github.com/router-for-me/CLIProxyAPI/v6/internal/buildinfo"
 	"github.com/router-for-me/CLIProxyAPI/v6/internal/config"
 	"github.com/router-for-me/CLIProxyAPI/v6/internal/usage"
 	sdkAuth "github.com/router-for-me/CLIProxyAPI/v6/sdk/auth"
@@ -91,6 +92,10 @@ func (h *Handler) Middleware() gin.HandlerFunc {
 	const banDuration = 30 * time.Minute
 
 	return func(c *gin.Context) {
+		c.Header("X-CPA-VERSION", buildinfo.Version)
+		c.Header("X-CPA-COMMIT", buildinfo.Commit)
+		c.Header("X-CPA-BUILD-DATE", buildinfo.BuildDate)
+
 		clientIP := c.ClientIP()
 		localClient := clientIP == "127.0.0.1" || clientIP == "::1"
 		cfg := h.cfg

internal/buildinfo/buildinfo.go

@@ -0,0 +1,15 @@
+// Package buildinfo exposes compile-time metadata shared across the server.
+package buildinfo
+
+// The following variables are overridden via ldflags during release builds.
+// Defaults cover local development builds.
+var (
+	// Version is the semantic version or git describe output of the binary.
+	Version = "dev"
+
+	// Commit is the git commit SHA baked into the binary.
+	Commit = "none"
+
+	// BuildDate records when the binary was built in UTC.
+	BuildDate = "unknown"
+)