feat: implement auth handler, middleware, and router

domain/context.go: extend WithUser/UserFromContext with session ID

handler/response.go: respondJSON/respondError with domain→HTTP status
  mapping (404/403/401/409/400/415/500)
handler/middleware.go: Bearer JWT extraction, ParseAccessToken,
  domain.WithUser injection; aborts with 401 JSON on failure
handler/auth_handler.go: Login, Refresh, Logout, ListSessions,
  TerminateSession
handler/router.go: /health, /api/v1/auth routes; login and refresh
  are public, session routes protected by AuthMiddleware

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/handler/router.go

@@ -0,0 +1,37 @@
+package handler
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+// NewRouter builds and returns a configured Gin engine.
+// Additional handlers will be added here as they are implemented.
+func NewRouter(auth *AuthMiddleware, authHandler *AuthHandler) *gin.Engine {
+	r := gin.New()
+	r.Use(gin.Logger(), gin.Recovery())
+
+	// Health check — no auth required.
+	r.GET("/health", func(c *gin.Context) {
+		c.JSON(http.StatusOK, gin.H{"status": "ok"})
+	})
+
+	v1 := r.Group("/api/v1")
+
+	// Auth endpoints — login and refresh are public; others require a valid token.
+	authGroup := v1.Group("/auth")
+	{
+		authGroup.POST("/login", authHandler.Login)
+		authGroup.POST("/refresh", authHandler.Refresh)
+
+		protected := authGroup.Group("", auth.Handle())
+		{
+			protected.POST("/logout", authHandler.Logout)
+			protected.GET("/sessions", authHandler.ListSessions)
+			protected.DELETE("/sessions/:id", authHandler.TerminateSession)
+		}
+	}
+
+	return r
+}