Add EUPHEMIA-compatible bid objects and CI workflow

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Summary

• Add comprehensive bid object models for European power market auctions
• Implement GitHub Actions CI workflow for automated quality checks
• Fix all ruff linting issues in test suite

Changes

Bid Objects (bids.py)

• SimpleHourlyBid: Price-quantity pairs per MTU with validation
• BlockBid: Fixed price/volume across consecutive MTUs
• LinkedBlockBid: Parent-child bid relationships with acceptance rules
• ExclusiveGroup: Mutually exclusive block bids with constraints
• Full Pydantic v2 models with strict validation
• EUPHEMIA-compatible output formats

CI/CD

• Automated testing on push and pull requests
• Ruff linting and formatting checks
• Mypy type checking (strict mode)
• Test coverage enforcement (80% threshold)
• Python 3.11 support
• Codecov integration

Code Quality

• Remove unused imports
• Fix import ordering
• Add strict parameter to zip() calls
• Fix line length violations (<100 chars)
• Update to datetime.UTC alias
• Rename unused loop variables

Test Coverage

• 149 tests passing
• 97% code coverage
• Property-based tests with hypothesis
• All bid types validated against EUPHEMIA requirements

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