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

apps/api/.env.example

@@ -65,6 +65,17 @@ SES_CONFIGURATION_SET_NO_TRACKING=plunk-configuration-set-no-tracking  # Optiona
 # Default: Fetched from AWS (typically 14 for sandbox, higher for production accounts)
 # EMAIL_RATE_LIMIT_PER_SECOND=14
 
+# Email worker concurrency (number of emails processed in parallel)
+# If not set, derived from the effective rate limit (~ rate * 0.5, min 5, capped
+# by EMAIL_WORKER_MAX_CONCURRENCY). Set this to pin a fixed value when the
+# Prisma connection pool or memory is the binding constraint.
+# EMAIL_WORKER_CONCURRENCY=10
+
+# Upper bound for auto-derived worker concurrency
+# Raise this when your SES quota is high AND the Prisma pool has been sized for it.
+# Default: 50
+# EMAIL_WORKER_MAX_CONCURRENCY=50
+
 # ==============================================================================
 # OAuth (Optional - for social login)
 # ==============================================================================