Add mutation testing with Stryker for service-layer robustness verification

[nanaf6203-bit]

Area: Test Quality
Priority: P3 — Low

Description

Code coverage doesn't catch all bugs. A test suite with 100% coverage can still miss logical errors. Mutation testing modifies source code to verify tests detect the change. Use Stryker for nightly mutation testing.

Acceptance Criteria

• Install @stryker-mutator/core, @stryker-mutator/typescript-checker, @stryker-mutator/jest-runner
• Configure stryker.conf.json targeting src/auth, src/tips, src/profiles service files
• Run mutation tests nightly (scheduled CI workflow, not on PRs)
• Target: mutation score > 70% on service files
• Document mutation testing policy and exclusions in CONTRIBUTING.md

Technical Notes

• Use concurrency: 4 for parallel mutants
• Timeout per mutant: 30s

[awwalmust]

Hi maintainer
I have reviewed this issue and would love to work on it, PR 2days
Thanks