fix: API routes with BackgroundTasks + proper request models

Add api __init__.py"

FIX: multi_agent_routes.py — add all missing imports and router declaration"

Add multi-agent API routes: iterative pipeline + behavior extraction + complexity + security scan

Add backend/app/api/frameworks.py — universal LLM provider + complete API

Add backend/app/api/generate.py — universal LLM provider + complete API