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
- The mapstructure library is no longer needed for direct duration
parsing since we now store TTLs as string durations (e.g., "15m",
"168h") and parse them on demand via helper methods.
- This allows more flexible duration formats in configuration and moves
the parsing responsibility to the JWT config struct itself.
