feat: add API key authentication and health endpoint

src/backend/main.py

@@ -0,0 +1,17 @@
+from contextlib import asynccontextmanager
+
+from fastapi import FastAPI
+
+from src.backend.api.routes.health import router as health_router
+from src.backend.db.database import init_db
+
+
+@asynccontextmanager
+async def lifespan(app: FastAPI):
+    await init_db()
+    yield
+
+
+app = FastAPI(title="OpusAgent", version="0.1.0", lifespan=lifespan)
+
+app.include_router(health_router, prefix="/api")