refactor(api): refactor API module with route definitions and response utilities

Restructure the core API module to improve clarity, consistency, and
maintainability:

- Introduce `defineApiRoutes()` helper for declarative route definitions
  with built-in config validation at startup
- Add `apiSuccess()` / `apiError()` response utilities; enforce their
  use across all handlers (core and modules)
- Move auth enforcement to route definitions (`auth: 'public' | 'user' |
  'admin'`), removing manual auth checks from handlers
- Extract core routes into `core-routes.js`; router now has no knowledge
  of specific features
- Rename `nx-route.js` to `route-handler.js` and update package.json
  export accordingly
- Update ARCHITECTURE.md to reflect new API conventions and point to
  `src/core/api/README.md` for details

docs/dev/COMMITS.md

@@ -0,0 +1,56 @@
+# Conventions de commit
+
+Tous les messages de commit sont rédigés en **anglais**, en suivant le format [Conventional Commits](https://www.conventionalcommits.org/) :
+
+```
+<type>(<scope>): <description courte>
+```
+
+---
+
+## Types
+
+| Type | Usage |
+|------|-------|
+| `feat` | New feature |
+| `fix` | Bug fix |
+| `refactor` | Code restructuring without behavior change |
+| `style` | Formatting only (spaces, commas, no logic change) |
+| `docs` | Documentation only |
+| `test` | Add or update tests |
+| `chore` | Maintenance, dependencies, build config |
+| `perf` | Performance improvement |
+| `revert` | Revert a previous commit |
+
+---
+
+## Exemples
+
+```
+feat(auth): add OAuth2 login support
+fix(api): handle null response from payment gateway
+refactor(storage): extract upload logic into helper
+docs(guide): add git commit message conventions
+chore(deps): update dependencies
+perf(db): cache user metadata on repeated reads
+revert: revert "feat(auth): add OAuth2 login support"
+```
+
+---
+
+## Règles
+
+- **Scope** : facultatif, précise la zone touchée `auth`, `api`, `storage`, `ui`, `config`
+- **Description** : minuscules, sans point final, en anglais
+- Un commit = une intention. Ne pas mélanger fix et refactor
+- Pas de `wip`, `fix fix`, `oups` ou messages vides
+
+## Breaking changes
+
+Ajouter `!` après le type et un pied de page `BREAKING CHANGE:` :
+
+```
+feat(api)!: remove legacy query parameter
+
+BREAKING CHANGE: the `legacy` param is no longer accepted, use `version` instead.
+```