feat: Add contributor reputation score service

[sojetunde8]

Closes #787

Summary

• Added a new reputationService that computes a 0–100 reputation score for any contributor based on completed bounties, dispute outcomes, and response times.
• Added GET /api/contributors/:address/reputation endpoint returning the aggregate score and per-factor breakdown.
• Scoring formula documented in code comments: base 50 + completion (0–60) + disputes (-30 to +20) + response time (0–20), clamped to 0–100.
• Contributors with no history receive a neutral score of 50.

Scope

Does not touch existing bounty lifecycle, leaderboard, or notification systems — out of scope for this issue.

Testing

• npx tsc --noEmit — passed (zero errors)
• npx eslint "src/**/*.ts" "test/**/*.ts" — passed (no new warnings; 7 pre-existing no-console warnings in other files)
• npx vitest run test/reputationService.test.ts — 6/6 tests passed
• npx vitest run test/openapi.snapshot.test.ts — snapshot updated and passes
• Full suite: 242/281 tests pass; 39 failures are all pre-existing (lockfile/data issues unrelated to this change)

Files changed

• backend/src/services/reputationService.ts — new service with scoring logic
• backend/src/app.ts — registered the new reputation endpoint
• backend/src/docs/openapi.ts — OpenAPI documentation for the new endpoint
• docs/openapi.generated.json — regenerated OpenAPI spec
• backend/test/reputationService.test.ts — unit tests for the service
• backend/test/__snapshots__/openapi.snapshot.test.ts.snap — updated snapshot

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