feat: add app factory with health endpoint and test infrastructure

src/fastapi_oidc_op/app.py

@@ -0,0 +1,23 @@
+from fastapi import FastAPI
+
+from fastapi_oidc_op.config import Settings
+
+
+def create_app(settings: Settings | None = None) -> FastAPI:
+    if settings is None:
+        settings = Settings()  # type: ignore[call-arg]
+
+    app = FastAPI(
+        title="FastAPI OIDC OP",
+        version="0.1.0",
+        docs_url="/docs" if settings.debug else None,
+        redoc_url=None,
+    )
+
+    app.state.settings = settings
+
+    @app.get("/health")
+    async def health() -> dict[str, str]:
+        return {"status": "ok"}
+
+    return app