Send CORS headers

[penguineer]

This pull request refactors the Tornado request handlers to consistently support Cross-Origin Resource Sharing (CORS) and improves error handling. The main change is the introduction of a reusable BaseCORSHandler, which sets CORS headers and is now used as the base class for several request handlers.

Fixes #15

CORS support and handler refactoring:

• Added a new BaseCORSHandler class that sets default CORS headers, handles HTTP OPTIONS requests, and ensures error responses also include CORS headers (src/app.py).
• Updated SpaceAPIHandler, SpaceStateTextHandler, and PictureHandler to inherit from BaseCORSHandler instead of directly from tornado.web.RequestHandler, ensuring these endpoints consistently support CORS (src/app.py).

[Copilot]

Pull Request Overview

This pull request adds CORS (Cross-Origin Resource Sharing) support to the Tornado web application by introducing a reusable base handler class and refactoring existing handlers to use it.

• Introduces BaseCORSHandler class that provides CORS headers, OPTIONS handling, and error response CORS support
• Updates three API handlers to inherit from the new CORS-enabled base class
• Adds typing import for better type annotations

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