feat(caching): add aggressive caching for expensive computations #603

[David-282]

Linked Issue

Closes #603

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What does this PR do?

Adds a dedicated ComputationCacheService for expensive O(n) operations in
LeaderboardService that were hitting the database on every request with no
caching. Implements cache-aside pattern with stampede protection, per-computation
TTL configuration, and Prometheus metrics tracking via the existing
MetricsCollectionService.

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Type of change

• ✨ New feature (non-breaking change that adds functionality)

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Pre-merge checklist (required)

Branch & metadata

• Branch name follows feature/issue-<N>-<slug> / fix/issue-<N>-<slug> convention
• Branch is up to date with the target branch
• All commits and the PR title follow the Conventional Commits format with issue reference

Code quality & tests

• npm run lint:ci
• npm run format:check
• npm run typecheck
• npm run test:ci
• New service methods have corresponding .spec.ts unit tests
• No existing tests were deleted

Error handling & NestJS best practices

• Logging goes through NestJS Logger with structured messages
• No new endpoints — authentication/authorization guards not applicable

API documentation / Swagger

• No API surface changes — this PR adds a service layer only, no new controller endpoints

Breaking changes

• This PR does not introduce a breaking API change

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Test evidence (required)

Commands run locally

Local tooling not fully set up (npm install pending).
CI pipeline will validate lint, format, typecheck, and tests on push.

Manual / API verification
No new endpoints introduced. Changes are internal service layer only.
Unit tests cover: cache hit, cache miss, stampede deduplication,
invalidation, hit rate calculation, and Prometheus metrics publishing.

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Files changed

• src/caching/computation-cache.service.ts — new service
• src/caching/computation-cache.service.spec.ts — unit tests
• src/caching/caching.constants.ts — added COMPUTATION_TTL
• src/caching/cache-key.builder.ts — added buildComputationKey()
• src/caching/caching.module.ts — registered and exported ComputationCacheService

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