test: increase unit test coverage to 85% for auth and critical modules

[walexjnr]

Summary

Increases unit test coverage across critical modules by adding comprehensive specs for AuthService and TokenBlacklistService — the two most undertested areas identified in the issue audit.

Changes:

src/auth/auth.service.spec.ts (new)

Full coverage of AuthService including:

• login — verifies token generation and refresh token hash storage
• logout — verifies refresh token nullification
• refreshTokens (error paths):
  • Invalid/expired token signature → UnauthorizedException
  • User not found → UnauthorizedException
  • User has no stored refresh token → UnauthorizedException
  • Blacklisted token reuse → revokes session + UnauthorizedException
• refreshTokens (happy path): issues new tokens, blacklists old JTI

src/auth/services/token-blacklist.service.spec.ts (new)

Full coverage of TokenBlacklistService including:

• addToBlacklist — correct cache key (bl_token:{jti}), TTL conversion ms→s, ceiling rounding
• isBlacklisted — returns true for 'revoked', false for null and any other value

Test Plan

• pnpm test — all new specs pass locally
• pnpm run test:cov — auth module coverage increases to ≥90%
• No lint warnings (pnpm run lint:ci)
• Build still passes (pnpm run build)

closes #514

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