fix: add server.js shim for backwards-compatible TypeScript execution
The Nomad entrypoint calls `exec node server.js` but the codebase
migrated to TypeScript at server/index.ts. Rather than updating every
deployment config simultaneously, add a server.js shim that registers
tsx and imports the TypeScript server.
This makes `node server.js` work everywhere without changing Nomad
job specs, Docker CMDs, or other deployment configs.