feat(infra): add Stalwart mail server, route mana-notify SMTP through it

Add self-hosted Stalwart mail server (Rust, ~50MB RAM) to replace Brevo
as SMTP provider. mana-notify now sends via stalwart:587 internally.

Ports exposed: 25 (SMTP), 587 (submission), 465 (SMTPS), 993 (IMAPS),
8443 (web admin). Requires DNS setup (MX, SPF, DKIM, DMARC) and router
port-forwarding to complete the migration.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.macmini.yml

@@ -535,9 +535,9 @@ services:
       DATABASE_URL: postgresql://postgres:${POSTGRES_PASSWORD:-mana123}@postgres:5432/mana_platform?sslmode=disable
       SERVICE_KEY: ${MANA_CORE_SERVICE_KEY:-dev-service-key}
       MANA_CORE_AUTH_URL: http://mana-auth:3001
-      SMTP_HOST: ${SMTP_HOST:-smtp-relay.brevo.com}
+      SMTP_HOST: ${SMTP_HOST:-stalwart}
       SMTP_PORT: ${SMTP_PORT:-587}
-      SMTP_USER: ${SMTP_USER:-94cde5002@smtp-brevo.com}
+      SMTP_USER: ${SMTP_USER:-noreply@mana.how}
       SMTP_PASSWORD: ${SMTP_PASSWORD:-}
       SMTP_FROM: "ManaCore <noreply@mana.how>"
       EXPO_ACCESS_TOKEN: ${EXPO_ACCESS_TOKEN:-}
@@ -552,6 +552,28 @@ services:
       retries: 3
       start_period: 5s
 
+  stalwart:
+    image: stalwartlabs/mail-server:latest
+    container_name: mana-mail
+    restart: always
+    mem_limit: 256m
+    ports:
+      - "25:25"
+      - "587:587"
+      - "465:465"
+      - "993:993"
+      - "8443:8443"
+    volumes:
+      - stalwart_data:/opt/stalwart-mail
+    environment:
+      - STALWART_ADMIN_PASSWORD=${STALWART_ADMIN_PASSWORD:-ChangeMe123!}
+    healthcheck:
+      test: ["CMD", "curl", "-f", "http://localhost:8080/healthz"]
+      interval: 120s
+      timeout: 5s
+      retries: 3
+      start_period: 10s
+
   mana-crawler:
     build:
       context: .
@@ -1550,3 +1572,5 @@ volumes:
     name: mana-loki-data
   matrix_bots_data:
     name: mana-matrix-bots-data
+  stalwart_data:
+    name: mana-stalwart-data