[Bug] Split API app construction from server startup

[LukastGeorge]

References #33 and #221.

/bounty $50

Problem

apps/api/src/index.ts builds the Express app and immediately calls app.listen(...) at module top level. That is fine for local npm run dev, but it makes the API entrypoint hard to import from tests or scripts because importing the module has a side effect: it binds a real port.

Current behavior couples app construction with process startup:

const app = express();
const port = process.env.PORT || 4000;

app.use(express.json());
// routes...

app.listen(port, () => {
  console.log(`TaskFlow API listening on port ${port}`);
});

Why it matters

A future route test suite should be able to import the Express app and call routes in memory. With the current structure, importing the module can cause:

• port conflicts when more than one test imports the module;
• hanging tests because the listener stays open;
• harder route-level test setup because app creation and process startup are coupled.

Expected behavior

The Express app should be exportable without starting a listener. Server startup should only happen in the dev/runtime entrypoint.

Suggested fix

Split app construction from server startup:

• apps/api/src/app.ts: creates/configures app, exports it.
• apps/api/src/index.ts: imports app and calls listen for runtime/dev usage only.

Acceptance criteria

• Importing the Express app does not bind a port.
• npm run dev -w apps/api still starts the API server normally.
• Existing /health and /users behavior stays unchanged.
• Add a small test or smoke script demonstrating the app can be imported without starting the listener.

[LukastGeorge]

/claim #1771

Submitted PR #1773: #1773

Scope: split Express app construction into an import-safe app module, keep runtime listen in index.ts, and add a focused Node test proving app import does not call listen.

Validation:

• npm test -w apps/api
• npm test
• contributors/agents.json parse check
• git diff --check

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