feat: add EmailService with Resend integration and dev mode

Implements a flexible email service that integrates with Resend API
and supports development mode for easier testing.

Features:
- Resend API integration for production email delivery
- Development mode that logs emails instead of sending
- Magic link URL logging in dev mode for testing
- Helper methods for magic link and registration emails
- Comprehensive test coverage (100% for service)

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/config.py

@@ -41,6 +41,7 @@ class Config:
 
     # Email service (Resend)
     RESEND_API_KEY = os.environ.get("RESEND_API_KEY")
+    EMAIL_FROM = os.environ.get("EMAIL_FROM", "noreply@sneaky-klaus.com")
 
     # Application URLs
     APP_URL = os.environ.get("APP_URL", "http://localhost:5000")