[Vladush]

Description

Implemented Phase 1 of performance and security optimizations:

1. Replaced std::regex: Switched username validation to a manual character loop (Fixes CodeQL ReDoS warning, strictly blocks path traversal).
2. GPU Throttle: Changed default gpu_throttle_ms to 20ms to prevent UI hangs.
3. Model Pre-loading: Models now load in AuthEngine::init() (service startup), eliminating first-login latency.

Type of change

• Bug fix (ReDoS, Path Traversal)
• New feature (Performance tuning: pre-loading, throttling)
• Breaking change
• Documentation update

How Has This Been Tested?

• linuxcampam_tests: Passed 30/30 tests (including new strict validation logic).
• Manual Verification: Verified config defaults and logs.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes