feat(docs): use parameterized OpenAPI server URL

Replace the static `https://sofa.example.com/api/v1` server entry with a variable-based URL (`{protocol}://{host}:{port}/api/v1`) so the spec works against any instance out of the box. Also move the output path from `docs/openapi.json` to `docs/public/openapi.json` so the file is served as a static asset by the docs site.

docs/src/lib/openapi.ts

@@ -1,5 +1,5 @@
 import { createOpenAPI } from "fumadocs-openapi/server";
 
 export const openapi = createOpenAPI({
-  input: ["./openapi.json"],
+  input: ["./public/openapi.json"],
 });