feat(EmailService): add worker concurrency settings and improve email queue prioritization

.env.self-host.example

@@ -163,6 +163,19 @@ SMTP_DOMAIN=smtp.example.com
 # Default: unset (auto-detect, falls back to 14)
 # EMAIL_RATE_LIMIT_PER_SECOND=1
 
+# Number of emails the worker processes in parallel. When unset, concurrency is
+# derived from the effective rate limit (~ rate * 0.5, min 5, capped by
+# EMAIL_WORKER_MAX_CONCURRENCY) so a higher SES quota translates into higher
+# throughput automatically. Pin this only when the Prisma pool or memory is the
+# binding constraint.
+# Default: unset (auto-derived)
+# EMAIL_WORKER_CONCURRENCY=10
+
+# Upper bound applied to the auto-derived concurrency. Raise this when your SES
+# quota is high AND the Prisma connection pool has been sized for it.
+# Default: 50
+# EMAIL_WORKER_MAX_CONCURRENCY=50
+
 # ========================================
 # ADVANCED (rarely needed)
 # ========================================