fix: batch 4 — connection pooling, graceful shutdown, email verification UX

- Prisma: connection_limit=10, pool_timeout=30 on DATABASE_URL in both compose files
- Graceful shutdown: SIGTERM/SIGINT forwarded to Node process in docker-entrypoint.sh
- testEmailConnection: replaced real email send with transporter.verify(), simplified UI to single button
- NotificationLog.userId index: confirmed already present, no change needed

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

prisma/schema.prisma

@@ -11,6 +11,10 @@ generator client {
 
 datasource db {
   provider = "postgresql"
+  // connection_limit and pool_timeout are set via query params in DATABASE_URL:
+  //   ?connection_limit=10&pool_timeout=30
+  // Defaults: connection_limit = num_cpus * 2 + 1, pool_timeout = 10s.
+  // Override in .env for production to prevent connection exhaustion.
   url      = env("DATABASE_URL")
 }