Add metric: average number of problems per division

[DebuggingMax]

Summary

Closes #100

This PR adds a new metric that calculates the average number of problems solved per contest for each division (Div. 1, Div. 2, Div. 3, Div. 4).

Changes

New Files

• backend/domain/models/division_problems.py - Domain models for division analysis
• backend/domain/services/division_problems_service.py - Service to analyze submissions by division
• backend/api/routes/division_problems.py - API route handler
• backend/api/schemas/division_problems.py - Pydantic schemas for API response
• backend/tests/unit/division_problems_service/ - Unit tests for the service
• backend/tests/unit/codeforces_client/test_extract_division.py - Tests for division extraction

Modified Files

• backend/infrastructure/codeforces_client.py - Added get_contest_divisions() method with division extraction from contest names
• backend/services/codeforces_data_service.py - Added method to expose contest divisions
• backend/api/deps.py - Added dependency provider for new service
• backend/api/routes/__init__.py - Registered new controller
• backend/domain/models/__init__.py - Exported new models
• backend/domain/services/__init__.py - Exported new service

API Endpoint

GET /api/division-problems/{handle}

Response Example

{
  "divisions": [
    {
      "division": "Div. 1",
      "contest_count": 5,
      "total_problems_solved": 8,
      "average_problems_per_contest": 1.6
    },
    {
      "division": "Div. 2",
      "contest_count": 20,
      "total_problems_solved": 65,
      "average_problems_per_contest": 3.25
    }
  ],
  "total_contests": 25,
  "total_problems_solved": 73,
  "last_updated": "2024-01-15T12:00:00Z"
}

Implementation Details

• Fetches contest list from Codeforces API to build a mapping of contest_id → division
• Caches the division mapping globally to avoid repeated API calls
• Extracts division from contest names using regex (handles "Div. 1", "Div. 2", etc.)
• For combined rounds (e.g., "Div. 1 + Div. 2"), uses the harder division (lower number)
• Follows existing patterns for caching, staleness handling, and async task queueing

@deyna256

[DebuggingMax]

Hey @deyna256! 👋

This PR implements the metric requested in #100 - average number of problems per division.

Key features:

• New endpoint /api/division-problems/{handle}
• Extracts division info from contest names (Div. 1-4)
• Calculates average problems solved per contest for each division
• Follows existing patterns (caching, staleness handling, async tasks)
• Includes comprehensive tests

Ready for review! Let me know if you'd like any changes.

[deyna256]

@DebuggingMax good!