═══════════════════════════════════════════════════════════════════════
  CHANGES TO YOUR EXISTING `mails` MODULE  (additions only — remove nothing)
═══════════════════════════════════════════════════════════════════════

This `email-settings/` folder is the NEW standalone module. Drop it under
`src/` so it sits next to your `mails/` folder:

    src/
      email-settings/   ← this folder
      mails/            ← your existing module

Then make 3 edits inside your existing mails module.

───────────────────────────────────────────────────────────────────────
1) mails.service.ts — add these 2 imports at the top
───────────────────────────────────────────────────────────────────────
import { EmailSettingsService } from '../email-settings/email-settings.service';
import { EmailSettingsEntity } from '../email-settings/email-settings.entity';

(If email-settings/ and mails/ aren't siblings, fix the relative paths.)

───────────────────────────────────────────────────────────────────────
2) mails.service.ts — add cache field + inject service in the constructor
───────────────────────────────────────────────────────────────────────
  private transporter: nodemailer.Transporter;

  // per-user transporters, keyed by config id
  private readonly userTransportCache = new Map<
    number,
    { transporter: nodemailer.Transporter; from: string; signature: string }
  >();

  constructor(
    private readonly emailSettings: EmailSettingsService,
  ) {
    this.transporter = this.createTransporter(); // unchanged
  }

───────────────────────────────────────────────────────────────────────
3) mails.service.ts — add these 2 methods before the final closing }
───────────────────────────────────────────────────────────────────────
  async sendAsUser(
    userId: number | null,
    opts: {
      to: string | string[];
      subject: string;
      html?: string;
      text?: string;
      cc?: string | string[];
      bcc?: string | string[];
      replyTo?: string;
      attachments?: nodemailer.SendMailOptions['attachments'];
    },
  ): Promise<void> {
    const cfg = userId ? await this.emailSettings.getRawForUser(userId) : null;

    if (!cfg) {
      await this.transporter.sendMail({ from: this.getFromAddress(), ...opts });
      this.logger.log(
        `Email sent via DEFAULT sender (user_id=${userId ?? 'none'}) — "${opts.subject}"`,
      );
      return;
    }

    const { transporter, from } = this.getUserTransport(cfg);
    try {
      await transporter.sendMail({ from, ...opts });
      this.logger.log(
        `Email sent as ${cfg.from_email} (user_id=${userId}) — "${opts.subject}"`,
      );
    } catch (err: any) {
      this.userTransportCache.delete(cfg.id);
      this.logger.error(`sendAsUser failed via ${cfg.from_email}: ${err.message}`);
      throw err;
    }
  }

  private getUserTransport(cfg: EmailSettingsEntity) {
    const signature = `${cfg.smtp_host}|${cfg.smtp_port}|${cfg.smtp_username}|${
      cfg.updated_at ? new Date(cfg.updated_at).getTime() : 0
    }`;
    const cached = this.userTransportCache.get(cfg.id);
    if (cached && cached.signature === signature) return cached;

    const port = Number(cfg.smtp_port);
    const enc = (cfg.smtp_encryption || '').toLowerCase();
    const secure = port === 465 || enc === 'ssl'; // 465=SSL, 587=STARTTLS

    const transporter = nodemailer.createTransport({
      host: cfg.smtp_host,
      port,
      secure,
      auth: { user: cfg.smtp_username, pass: cfg.smtp_password },
      ...(secure ? {} : { requireTLS: true }),
    });

    const name = (cfg.from_name || '').trim();
    const from = name ? `"${name}" <${cfg.from_email}>` : cfg.from_email;

    const entry = { transporter, from, signature };
    this.userTransportCache.set(cfg.id, entry);
    return entry;
  }

───────────────────────────────────────────────────────────────────────
4) mails.module.ts — import EmailSettingsModule
───────────────────────────────────────────────────────────────────────
import { Module } from '@nestjs/common';
import { MailsService } from './mails.service';
import { EmailSettingsModule } from '../email-settings/email-settings.module';

@Module({
  imports: [EmailSettingsModule],
  providers: [MailsService],
  exports: [MailsService],
})
export class MailsModule {}

───────────────────────────────────────────────────────────────────────
5) app.module.ts — register the new module
───────────────────────────────────────────────────────────────────────
Add EmailSettingsModule to your AppModule `imports: [...]` so its controller
(/email-settings routes) is active.

───────────────────────────────────────────────────────────────────────
BEFORE IT RUNS — confirm:
  • The 'scheme_dbs' connection name (in email-settings.entity.ts injection
    and email-settings.module.ts forFeature) is the DB that actually holds
    the email_settings table. Your screenshot shows it in `tqs`.
  • email_settings.user_id values match the userId you pass to sendAsUser().
  • nodemailer is installed:  npm i nodemailer  (+ npm i -D @types/nodemailer)