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,
+		},
+	})
+}