Add challenge voting endpoint

[newmattock]

Summary

• Add a NestJS challenges module with POST /challenges/:challengeId/votes and GET /challenges/:challengeId/votes.
• Store one up/down vote per user per challenge and update repeated votes without double-counting.
• Register the module in the backend app and repair existing module/type issues that blocked full TypeScript validation.

Verification

• node node_modules/jest/bin/jest.js challenges --runInBand
• node node_modules/typescript/bin/tsc --noEmit --pretty false
• npm test -- challenges --runInBand is blocked locally by the repo's tracked non-executable node_modules/.bin/jest shim, so the direct Node invocation above was used.

Closes #175

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[MaryammAli]

LGTM