Implement web API foundation

Add application container, Gin router, graceful shutdown handler,
and version endpoint. This establishes the skeleton for the WebDisk
HTTP API as described in the architecture.

- Add internal/app/WebApp for runtime dependencies and version
- Add internal/server/router with GET /api/v1/version route
- Add graceful shutdown runner with signal handling in cmd/serve
- Add internal/api/ErrorResponse for standard HTTP error body
- Update roadmap, architecture, and decisions documentation

internal/api/response.go

@@ -0,0 +1,24 @@
+package api
+
+import (
+	"github.com/gin-gonic/gin"
+)
+
+// ErrorResponse is the standard JSON body for HTTP API errors.
+type ErrorResponse struct {
+	Error ErrorBody `json:"error"`
+}
+
+// ErrorBody contains human-readable error details.
+type ErrorBody struct {
+	Message string `json:"message"`
+}
+
+// Error writes a JSON error response.
+func Error(c *gin.Context, status int, message string) {
+	c.JSON(status, ErrorResponse{
+		Error: ErrorBody{
+			Message: message,
+		},
+	})
+}

internal/api/response_test.go

@@ -0,0 +1,36 @@
+package api
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+)
+
+func TestError(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	router := gin.New()
+	router.GET("/error", func(c *gin.Context) {
+		Error(c, http.StatusBadRequest, "invalid request")
+	})
+
+	req := httptest.NewRequest(http.MethodGet, "/error", nil)
+	rec := httptest.NewRecorder()
+
+	router.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusBadRequest {
+		t.Fatalf("status = %d, want %d", rec.Code, http.StatusBadRequest)
+	}
+
+	var body ErrorResponse
+	if err := json.Unmarshal(rec.Body.Bytes(), &body); err != nil {
+		t.Fatalf("unmarshal response: %v", err)
+	}
+
+	if body.Error.Message != "invalid request" {
+		t.Errorf("message = %q, want %q", body.Error.Message, "invalid request")
+	}
+}