Add structured logging with zerolog

- Implement zerolog logger package with configurable levels
- Add contextual logging middleware (request_id, user_id, method, path, status, duration)
- Support both JSON (production) and human-readable (development) formats
- Add LOG_LEVEL and ENVIRONMENT configuration variables
- Replace standard log calls with structured logger throughout application
- Integrate logger in HTTP router and auth middleware

.env.example

@@ -25,3 +25,7 @@ SEND_WELCOME_EMAIL=false
 
 # Registration Control
 REGISTRATION_MODE=open
+
+# Logging Configuration
+LOG_LEVEL=info
+ENVIRONMENT=production