feat: add KNN-based user collaborative filtering (Issue #51)

[jahnavichalla19-Jaanu]

closes #51

• Add knn_collaborative.py with KNNCollaborativeRecommender
• Implements cosine similarity between users via sklearn
• Finds top-K similar users and recommends their highly rated items
• Adds method=knn query param to GET /api/recommend endpoint
• Add tests/test_knn_collaborative.py with 15 unit tests

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