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/handler/version.go

@@ -0,0 +1,27 @@
+package handler
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+// VersionResponse is the response body for the version endpoint.
+type VersionResponse struct {
+	Version string `json:"version"`
+}
+
+// VersionHandler serves application version metadata.
+type VersionHandler struct {
+	version string
+}
+
+// NewVersionHandler creates a version handler.
+func NewVersionHandler(appVersion string) *VersionHandler {
+	return &VersionHandler{version: appVersion}
+}
+
+// Get writes the current application version.
+func (h *VersionHandler) Get(c *gin.Context) {
+	c.JSON(http.StatusOK, VersionResponse{Version: h.version})
+}