Add transactional emails: welcome, professional submission, status changes

- mail/mail.service.ts: MailService with HTML templates (welcome, submitted,
  approved, rejected, pending, deactivated). Silent if SMTP not configured.
- mail/mail.module.ts: global module so all services can inject MailService
- auth.service.ts: send welcome email on register()
- professionals.service.ts: notify admin on new submission; notify professional
  on approve/deny/deactivate/setPending

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/auth/auth.service.ts

@@ -3,6 +3,7 @@ import { JwtService } from '@nestjs/jwt';
 import * as bcrypt from 'bcryptjs';
 import { PrismaService } from '../prisma/prisma.service';
 import { SmsService } from '../sms/sms.service';
+import { MailService } from '../mail/mail.service';
 
 @Injectable()
 export class AuthService {
@@ -10,6 +11,7 @@ export class AuthService {
     private prisma: PrismaService,
     private jwt: JwtService,
     private sms: SmsService,
+    private mail: MailService,
   ) {}
 
   async register(email: string, password: string, name: string) {
@@ -21,6 +23,7 @@ export class AuthService {
       data: { email, password_hash, name },
     });
 
+    this.mail.sendWelcome(name, email).catch(() => {});
     return this.generateToken(user);
   }