Add plagiarism detection for contest submissions

[SeifMohmmed]

Changes:

• Added GET /contests/{contestId}/plagiarisms endpoint to retrieve plagiarism cases with problem and similarity threshold filtering.
• Added POST /plagiarisms/similarity endpoint to compare two code snippets and calculate similarity.
• Introduced IPlagiarismService abstraction and registered its implementation through dependency injection.
• Added plagiarism-related DTOs and mapping extensions for API responses.
• Implemented query, handler, and response models for retrieving contest plagiarism cases.
• Extended submission repository to fetch accepted contest submissions for plagiarism analysis.
• Added CodeSimilarityRequest model for similarity checks.
• Enhanced contest integrity by providing automated plagiarism detection and reporting capabilities.

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