feat(deps): adopt p-limit for unbounded mass-op fan-outs

Cap concurrency on two services that were fanning out unbounded
requests to external systems:

1. email-compose.service.ts — attachment resolution. User attaches
   20 files → 20 simultaneous S3/MinIO GETs + 20 buffers in heap.
   Now capped at 4 concurrent reads; peak memory bounded by
   4 × max-attachment-size regardless of attachment count.

2. document-signing-emails.service.ts — sendSigningCompleted fanned
   out one SMTP send per recipient simultaneously. A Sales Contract
   with 10 recipients (client + 5 sellers + 4 witnesses) hit SMTP
   provider connection limits (Mailgun/SES/Postmark all cap concurrent
   connections in the single digits) and dropped overflow silently.
   Now capped at 3 concurrent sends.

Both use `pLimit(N)` from the Sindre Sorhus suite — well-tested at
scale, ~1kb gzip per service. Pattern is established for the
remaining audit-flagged mass-op services (brochures, backup, GDPR
export) to adopt as those files are touched.

Verified: tsc clean, vitest 1293/1293 pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/services/document-signing-emails.service.ts

@@ -28,6 +28,8 @@
  * port (single-tenant deploys can keep using Documenso's hosted UI).
  */
 
+import pLimit from 'p-limit';
+
 import { sendEmail } from '@/lib/email';
 import { getBrandingShell } from '@/lib/email/branding-resolver';
 import {
@@ -231,33 +233,41 @@ export async function sendSigningReminder(args: SigningReminderArgs): Promise<vo
 export async function sendSigningCompleted(args: SigningCompletedArgs): Promise<void> {
   const branding = await getBrandingShell(args.portId);
 
+  // Cap concurrency at 3: a Sales Contract with 10 recipients (client +
+  // 5 sellers + 4 witnesses) shouldn't fan out 10 simultaneous SMTP
+  // sends. Most SMTP providers (Mailgun, SES, Postmark) cap concurrent
+  // connections in the single digits and silently drop the overflow.
+  const sendLimit = pLimit(3);
+
   await Promise.all(
-    args.recipients.map(async (recipient) => {
-      const { subject, html, text } = signingCompletedEmail(
-        {
-          recipientName: recipient.name,
-          documentLabel: args.documentLabel,
-          clientName: args.clientName,
-          portName: args.portName,
-          completedAt: args.completedAt,
-        },
-        { branding },
-      );
-      try {
-        await sendEmail(recipient.email, subject, html, undefined, text, args.portId, [
-          { fileId: args.signedPdfFileId, filename: args.signedPdfFilename },
-        ]);
-        logger.info(
-          { portId: args.portId, recipient: recipient.email, documentLabel: args.documentLabel },
-          'Signing-completed email sent',
+    args.recipients.map((recipient) =>
+      sendLimit(async () => {
+        const { subject, html, text } = signingCompletedEmail(
+          {
+            recipientName: recipient.name,
+            documentLabel: args.documentLabel,
+            clientName: args.clientName,
+            portName: args.portName,
+            completedAt: args.completedAt,
+          },
+          { branding },
         );
-      } catch (err) {
-        logger.error(
-          { err, portId: args.portId, recipient: recipient.email },
-          'Signing-completed email send failed',
-        );
-        // Don't throw — sending to one recipient shouldn't block the others.
-      }
-    }),
+        try {
+          await sendEmail(recipient.email, subject, html, undefined, text, args.portId, [
+            { fileId: args.signedPdfFileId, filename: args.signedPdfFilename },
+          ]);
+          logger.info(
+            { portId: args.portId, recipient: recipient.email, documentLabel: args.documentLabel },
+            'Signing-completed email sent',
+          );
+        } catch (err) {
+          logger.error(
+            { err, portId: args.portId, recipient: recipient.email },
+            'Signing-completed email send failed',
+          );
+          // Don't throw — sending to one recipient shouldn't block the others.
+        }
+      }),
+    ),
   );
 }