feat: hot-reload dev server on Python file changes

[cristipufu]

Summary

• File watcher (watchfiles) monitors .py files in cwd and sends a WebSocket reload event to all connected clients
• Frontend shows a toast prompting the user to reload; clicking it calls POST /api/reload which flushes user modules from sys.modules and recreates the runtime factory
• Graph data is snapshotted per run at creation time and persisted on the backend, so existing runs keep their graph across reloads and browser refreshes
• Reload-pending state is tracked server-side so the toast persists across page refreshes
• Bumps version to 0.0.41

Test plan

• Start dev server with a factory_creator lambda
• Modify a .py file — verify toast appears
• Click Reload — verify POST /api/reload succeeds and toast dismisses
• Create a new run — verify it uses updated code
• Verify existing runs keep their original graph after reload
• Refresh browser with pending reload — verify toast reappears
• Verify reload is blocked while a run is in progress (409)
• Dismiss toast with X button — verify it hides

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