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>

package.json

@@ -81,6 +81,7 @@
     "next-themes": "^0.4.6",
     "nodemailer": "^8.0.7",
     "openai": "^6.37.0",
+    "p-limit": "^7.3.0",
     "pdf-lib": "^1.17.1",
     "pdfkit": "^0.18.0",
     "pino": "^10.3.1",