chore: initial project setup with backend, frontend, CI/CD, and documentation

Add complete NexaFabric project structure including:
- FastAPI backend with SQLAlchemy models, JWT auth, RBAC, audit logging, and provider interfaces
- React + TypeScript frontend with Vite, Tailwind CSS, TanStack Query, and Zustand
- Docker Compose configuration for PostgreSQL, Redis, API, worker, frontend, and nginx
- GitHub Actions and GitLab CI workflows for testing, linting, building, and security scanning
- Environment

backend/app/db/session.py

@@ -0,0 +1,25 @@
+from collections.abc import Generator
+
+from sqlalchemy import create_engine
+from sqlalchemy.orm import DeclarativeBase, Session, sessionmaker
+
+from app.core.config import get_settings
+
+
+class Base(DeclarativeBase):
+    pass
+
+
+settings = get_settings()
+connect_args = {"check_same_thread": False} if settings.database_url.startswith("sqlite") else {}
+engine = create_engine(settings.database_url, pool_pre_ping=True, connect_args=connect_args)
+SessionLocal = sessionmaker(bind=engine, autoflush=False, autocommit=False, expire_on_commit=False)
+
+
+def get_db() -> Generator[Session, None, None]:
+    db = SessionLocal()
+    try:
+        yield db
+    finally:
+        db.close()
+