feat: configure SMTP email for Joplin Server

Added email configuration to Joplin Server using existing SMTP settings from .env:
- MAILER_ENABLED: 1
- MAILER_HOST, MAILER_PORT from EMAIL_SMTP_* variables
- MAILER_AUTH_USER and MAILER_AUTH_PASSWORD from .env
- MAILER_NOREPLY_EMAIL using EMAIL_FROM

This enables password reset emails and user notifications.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

joplin/compose.yaml

@@ -15,6 +15,14 @@ services:
       POSTGRES_DATABASE: ${JOPLIN_DB_NAME}
       POSTGRES_USER: ${DB_USER}
       POSTGRES_PASSWORD: ${DB_PASSWORD}
+      MAILER_ENABLED: 1
+      MAILER_HOST: ${EMAIL_SMTP_HOST}
+      MAILER_PORT: ${EMAIL_SMTP_PORT}
+      MAILER_SECURE: 1
+      MAILER_AUTH_USER: ${EMAIL_SMTP_USER}
+      MAILER_AUTH_PASSWORD: ${EMAIL_SMTP_PASSWORD}
+      MAILER_NOREPLY_NAME: Joplin Server
+      MAILER_NOREPLY_EMAIL: ${EMAIL_FROM}
     networks:
       - compose_network
     depends_on: