feat: Add custom exceptions and enhanced config validation

[ZaviQ7]

Summary

This PR improves the robustness of the MiroFish backend by implementing two key architectural improvements:

1. Custom Exception Hierarchy

• Created a MiroFishError base class with error codes, severity levels, and HTTP status codes.
• Added domain-specific exceptions for Configuration, Graphs, Simulations, and External APIs to replace generic Exception catches.

2. Enhanced Configuration Validation

• Implemented Config.validate_comprehensive() to check for valid URL formats, numeric ranges, and placeholder API keys.
• Added directory permission checks and a formatted startup validation report.

Testing

• Created unit tests in backend/tests/test_exceptions.py and backend/tests/test_config_validated.py.
• Verified all 15+ test cases pass locally.
• Ensured 100% backward compatibility with existing code.