add model selection backend support

[chinmaychahar]

Summary

Add backend support for user-selectable LLM models (Issue - #87).

• Dynamic model discovery via GET /models endpoint
• Optional model field on POST /generate-note to override the server default
• preferred_model persisted in user settings (MongoDB)
• Graceful fallback to server default when provider API is unreachable

Testing

• I have tested these changes locally
• I have run all relevant automated tests
• I have verified this does not break existing workflows
• For changes that can be tested in UI, I have included screenshots or gif animations of the changes.

How I Tested

• I ran make test and all backend tests pass
• Added new test file backend/tests/llm_providers/test_model_selection.py
• Verified GET /models endpoint returns expected shape via test client
• Verified preferred_model field persists and returns correctly in user settings endpoints

[linux-foundation-easycla]

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• ✅ login: chinmaychahar / name: chinmaychahar (2864580)