Refactor single-file codebase into modular structure with clear separation of concerns

[Copilot]

This PR addresses the issue of having all functionality contained in a single 147-line server.py file by refactoring it into a clean, modular architecture that follows the Single Responsibility Principle.

What Changed

Before: Single monolithic file (server.py) with mixed concerns

• API routes, business logic, data models, utilities all in one file
• Hard to maintain, test, and extend
• 147 lines of tightly coupled code

After: Modular architecture with 16 well-organized files

• Clear separation of concerns across dedicated modules
• 354 lines total, but much better organized and maintainable
• Both old and new versions work simultaneously for backward compatibility

New Architecture

├── main.py              # Application entry point
├── config/              # Configuration management
│   ├── settings.py      # Settings loading and validation
├── models/              # Data models and schemas  
│   ├── entities.py      # Business entities (SchoolEvent, SchoolSchedule)
│   ├── schemas.py       # API request/response models (SchoolSearch)
├── services/            # Business logic services
│   ├── neis_service.py  # NEIS API interaction
│   ├── ics_service.py   # ICS file conversion logic
│   ├── school_service.py # School data management
│   ├── cache_service.py # File caching logic
├── utils/               # Utility functions
│   ├── date_utils.py    # Date parsing/formatting
│   ├── file_utils.py    # File system operations
└── api/                 # API routes and handlers
    └── routes.py        # FastAPI endpoint definitions

Key Benefits

• Maintainability: Each module has a single, clear responsibility
• Testability: Individual components can be tested in isolation
• Extensibility: New features can be added without touching unrelated code
• Readability: Code is organized logically by functionality
• Backward Compatibility: Original server.py preserved and still functional

Usage

Both versions work identically:

# New modular version
uvicorn main:app --host 0.0.0.0 --port 8000

# Original version (still works)
uvicorn server:app --host 0.0.0.0 --port 8000

The API endpoints, functionality, and behavior remain exactly the same. This is purely an internal code organization improvement that makes the codebase more professional and maintainable.

Testing

• ✅ All modules import correctly
• ✅ Both old and new versions start successfully
• ✅ HTTP endpoints respond correctly (200 OK)
• ✅ All original functionality preserved
• ✅ Configuration loading works
• ✅ Date utilities function properly

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