Feature/149 email notification adapter

[mijinummi]

📧 Implement Provider-Agnostic Email Notification Adapter

Context

The platform requires a flexible and safe email notification system to support booking and payment events without coupling the application to a single email provider. This PR introduces a provider-agnostic email adapter with templated emails and a mock provider for development and testing environments.

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✨ What’s Included

🔌 Provider-Agnostic Email Adapter

• Unified sendEmail() API for system notifications
• Adapter-based design allowing easy provider swapping
• Clean abstraction to support SendGrid, SES, Mailgun, etc.

🧪 Mock Provider (Dev & Test Safe)

• Mock email provider that logs email content instead of sending
• Prevents accidental email sends in development and CI
• Ideal for local testing and automated tests

📨 Templated Emails

• Handlebars-based template rendering
• Booking-related email templates
• Payment-related email templates
• Dynamic data injection via template variables

⚙️ Environment-Based Configuration

• Email provider selected using environment variables
• Zero code changes required to switch providers

📊 Logging & Observability

• Logs successful email sends
• Logs failures with error context
• Improves debugging and operational visibility

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🧪 Test Coverage

• Email adapter behavior verified via unit tests
• Template rendering tested independently
• Ensures adapter API remains provider-agnostic

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✅ Acceptance Criteria Checklist

• Booking email templates implemented
• Payment email templates implemented
• Provider-agnostic email adapter
• Mock provider for development/testing
• Environment-based configuration
• Basic logging for send results and failures
• Unit tests for adapter and template rendering

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🔮 Future Extensions

This implementation enables:

• Adding real providers (SendGrid, SES, Mailgun) without refactor
• Event-driven notifications (booking/payment hooks)
• Expansion to SMS or push notifications using the same pattern

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Notes for Reviewers

• No breaking changes to existing modules
• Safe-by-default behavior in non-production environments
• Designed for extensibility and maintainability
  Closes Email Notification Adapter #149