Migrate backend to Gemini 3.1 Flash Lite and optimize JS/TS rulesets

refactor(backend): use asyncio and thread pools for background tasks, optimize scanner AST utils and checks

security: add security headers middleware, upgrade package dependencies, and resolve scanner false positives

fix: bypass Semgrep validate check and set scan timeouts (25s per file, 200s per batch)

feat: implement dynamic Semgrep rules sync engine, secure admin endpoint and offline rulesets