feat(api): add app factory and healthz endpoint

app/main.py

@@ -0,0 +1,19 @@
+from fastapi import FastAPI
+
+
+def create_app() -> FastAPI:
+    app = FastAPI(
+        title="ContextQuery AI",
+        version="0.1.0",
+        docs_url="/docs",
+        redoc_url=None,
+    )
+
+    @app.get("/healthz", tags=["system"])
+    def healthz() -> dict[str, str]:
+        return {"status": "ok"}
+
+    return app
+
+
+app = create_app()