feat: allow loading mods without a Steam API key

Add fetchModFromUrl/fetchModFileFromUrl and accept a fileUrl query param on /items routes so the frontend can download saves directly from a search result's file_url, with no key required.

packages/tts/src/index.ts

@@ -33,16 +33,16 @@ interface SteamResponse {
  */
 export async function fetchMod(id: string, apiKey: string): Promise<TTSMod> {
   const fileUrl = await getFileUrl(id, apiKey);
+  return fetchModFromUrl(fileUrl);
+}
 
-  const saveRes = await fetch(fileUrl);
-  if (!saveRes.ok) {
-    throw new SteamApiError(
-      `Failed to download save file (${saveRes.status})`,
-      502,
-    );
-  }
-
-  const buffer = await saveRes.arrayBuffer();
+/**
+ * Download a TTS save from a direct URL and BSON-deserialize it.
+ *
+ * @param fileUrl Direct URL to the save file (e.g. from a search result).
+ */
+export async function fetchModFromUrl(fileUrl: string): Promise<TTSMod> {
+  const buffer = await downloadSave(fileUrl);
   return deserialize(new Uint8Array(buffer)) as TTSMod;
 }
 
@@ -70,7 +70,23 @@ export async function fetchModFile(
   apiKey: string,
 ): Promise<{ data: ArrayBuffer; filename: string }> {
   const fileUrl = await getFileUrl(id, apiKey);
+  return fetchModFileFromUrl(fileUrl);
+}
 
+/**
+ * Fetch the raw save file bytes from a direct URL.
+ * Returns the bytes plus a derived filename.
+ */
+export async function fetchModFileFromUrl(
+  fileUrl: string,
+): Promise<{ data: ArrayBuffer; filename: string }> {
+  const data = await downloadSave(fileUrl);
+  const filename = getFileName(fileUrl, null);
+  return { data, filename };
+}
+
+/** Download a save file's bytes from a URL, throwing on failure. */
+async function downloadSave(fileUrl: string): Promise<ArrayBuffer> {
   const res = await fetch(fileUrl);
   if (!res.ok) {
     throw new SteamApiError(
@@ -78,10 +94,7 @@ export async function fetchModFile(
       502,
     );
   }
-
-  const data = await res.arrayBuffer();
-  const filename = getFileName(fileUrl, res.headers.get('content-disposition'));
-  return { data, filename };
+  return res.arrayBuffer();
 }
 
 /** Resolve the `file_url` for a Workshop item via the Steam API. */