Add React Email template support via Node.js microservice

[dnplkndll]

Summary

Adds React Email template support as a new template type (campaign_react) alongside existing Go templates and visual editor.

• New react-email-service/ Node.js microservice for rendering React Email templates
• HTTP client in Go to call the render service
• Database migration adding campaign_react to template_type enum
• Frontend support for selecting React templates
• Docker Compose configuration (optional, disabled by default)

Upstream proposal: knadh#2890

Architecture

┌─────────────────────┐         ┌──────────────────────┐
│   listmonk (Go)     │  HTTP   │  React Email Service │
│                     │ ──────► │  (Node.js/Fastify)   │
│  - Campaign Manager │  POST   │                      │
│  - Template storage │ /render │  - @react-email/*    │
└─────────────────────┘         └──────────────────────┘

Files Changed

New:

• react-email-service/ - Complete Node.js service with 3 sample templates
• internal/reactrender/client.go - Go HTTP client
• internal/migrations/v6.1.0.go - Database migration

Modified:

• models/templates.go - Add TemplateTypeCampaignReact constant
• internal/manager/manager.go - Add reactClient field
• internal/manager/message.go - Add React rendering path
• cmd/init.go, cmd/main.go, cmd/templates.go - Initialization and validation
• config.toml.sample - Add [react_email] section
• docker-compose.yml - Add optional react-email service
• frontend/src/views/TemplateForm.vue - UI for React template selection

Test plan

• Start react-email-service: cd react-email-service && npm run dev
• Verify health: curl http://localhost:3001/api/health
• Test render endpoint with sample payload
• Configure listmonk with react_email.enabled = true
• Create campaign with React template type
• Send test email and verify HTML output

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