feat: implement environment validation schema and test setup infrastureucture.

[dot-enny]

Closes #831

Summary of Changes

1. New Branch Created:

   • Created and switched to feature/831-configurable-bcrypt-rounds.

2. Configurable Cost Factor (src/auth/auth.service.ts):

   • Injected ConfigService into AuthService.
   • Replaced hardcoded bcrypt.genSalt(10) with bcrypt.genSalt(Number(this.configService.get('BCRYPT_ROUNDS', 12))).

3. Startup Environment Validation (src/config/env.validation.ts & scripts/validate-env.js):

   • Updated BCRYPT_ROUNDS schema rule to ensure cost factor is between 10 and 14 (min(10).max(14)), defaulting to 12.

4. Environment Template Updated (.env.example):

   • Updated BCRYPT_ROUNDS documentation to reflect the OWASP recommended cost factor (12) and default value.

5. Unit Tests and Performance (src/auth/auth.service.spec.ts, src/config/env.validation.spec.ts, test/setup.ts, test/setup.js):

   • Set BCRYPT_ROUNDS=10 in test setup and mock configuration for fast execution during testing.
   • Added unit test assertions verifying that values < 10 or > 14 fail validation, and that default value 12 is applied.
   • All unit tests pass cleanly.

All tests have passed successfully. The solution is complete and verified.

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