feat(release): Backend Gestionnaire Creation (#17)

- Implemented MailModule and MailService
- Updated GestionnairesService to send welcome email
- Forced password change on first login for new gestionnaires

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/src/modules/mail/mail.module.ts

@@ -0,0 +1,10 @@
+import { Module } from '@nestjs/common';
+import { MailService } from './mail.service';
+import { AppConfigModule } from '../config/config.module';
+
+@Module({
+  imports: [AppConfigModule],
+  providers: [MailService],
+  exports: [MailService],
+})
+export class MailModule {}