fix(auth): OAuth GET callback crashes on Express 5 (req.body undefined)

[PierreBrisorgueil]

Symptom

After enabling Google OAuth on a downstream project (trawl), hitting the callback URL returns:

```
{"message":"Cannot read properties of undefined (reading 'strategy')"}
```

URL: `GET /api/auth/google/callback?code=...&scope=...&state=...`

Root cause

`modules/auth/controllers/auth.controller.js:345`

```js
if (req.body.strategy === false && req.body.key) {
```

Express 5 (we're on `^5.2.1`) leaves `req.body` as `undefined` when no body-parser middleware has processed the request. GET callbacks from Google have no body → `req.body.strategy` throws before reaching `passport.authenticate()`.

Express 4 used to initialize `req.body` to `{}`; Express 5 removed this.

Impact

• Blocks Google OAuth entirely. Every signin attempt throws.
• Does NOT affect Apple (Apple uses POST `form_post` response mode → body populated).

Fix

Optional-chain the body access:

```js
if (req.body?.strategy === false && req.body?.key) {
```

1 line. Also add `ERRORS.md` entry documenting the Express 5 `req.body` regression pattern.

Tests

Existing `auth.integration.tests.js` exercises POST callback (line 569). Add a GET callback test to prevent regression.