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/app/version.go

@@ -0,0 +1,4 @@
+package app
+
+// Version is the application version. Release builds can override it with ldflags.
+var AppVersion = "dev"