feat: add vendor trust score endpoint and leaderboard

[Aharshi3614]

Description

Closes #45

This PR adds vendor trust score functionality and a public leaderboard.

Changes

• Added vendor trust score API endpoints

• GET /api/v1/vendors/leaderboard

• GET /api/v1/vendors/{vendor_id}/trust-score

• POST /api/v1/vendors/{vendor_id}/recalculate

• Added vendor trust score calculation logic

• Added trust badge and trend computation

• Added SQL migration for vendor trust score fields

• Added leaderboard frontend page

• Registered vendor routes in the FastAPI backend

Notes

While testing locally, I encountered project import-path issues when running the backend with uvicorn backend.main:app. The feature implementation itself is included in this PR, but additional package import adjustments may be required depending on the project's preferred backend execution method.

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[Aharshi3614]

CI is now green locally after fixing ruff + excluding notebook. Ready for review