refactor: complete configuration cleanup and consistency fixes

- Create docker-compose.dev.yml with Mailhog for development email testing
- Standardize all configurations to use PORT=3001 for backend
- Fix database service naming (postgres → db) across all files
- Add missing BACKEND_URL environment variable to all configs
- Update .env examples to match actual Docker setup requirements
- Remove orphaned postgres-init directory (Umami handles its own DB)
- Update README roadmap: mark gallery feedback as implemented, add multi-admin support
- Update deployment guide with development setup instructions
- Fix frontend Dockerfile.dev for proper hot-reload development
- Remove unused files (wedding-photos.db, frontend/README.md)

This ensures all configuration files are consistent and aligned with the deployment guide.

backend/.env.example

@@ -12,6 +12,7 @@ JWT_SECRET=your-very-secure-jwt-secret-at-least-32-characters-long-example123456
 # URLs (adjust for your domain)
 ADMIN_URL=https://photos.example.com
 FRONTEND_URL=https://photos.example.com
+BACKEND_URL=https://photos.example.com  # Or https://api.photos.example.com if separate
 
 # Database Configuration
 DATABASE_CLIENT=pg