feat: import a Calibre library

Reads metadata.db and copies the books into a library — from a zip uploaded on
the library settings page, or from a path with `litestar calibre-import`. The
source is never touched, and re-running only picks up what is new.

Also names the formats mimetypes does not know: a Calibre library is full of
MOBI and AZW3, and a null content type used to fail the book endpoint.

frontend/src/routes/api/[...path]/+server.ts

@@ -30,6 +30,20 @@ async function handleResponse(response: Response) {
 	}
 }
 
+/**
+ * The body to forward, as fetch init options.
+ *
+ * Passed through as a stream rather than read into memory first: this process should
+ * never hold a whole upload, which for a zipped Calibre library could be many
+ * gigabytes. A request with no body contributes nothing, since `duplex` without a body
+ * is rejected.
+ */
+function bodyOf(request: Request) {
+	if (!request.body) return {};
+
+	return { body: request.body, duplex: 'half' } as RequestInit;
+}
+
 // Shared function to prepare the request with authentication
 function prepareRequest(locals: App.Locals, request: Request) {
 	const token = locals.authToken || 'server-default-token';
@@ -76,13 +90,14 @@ export const POST: RequestHandler = async ({ params, locals, fetch, request, url
 		const backendUrl = `${BACKEND_API_URL}/${path}${queryString}`;
 		const headers = prepareRequest(locals, request);
 
-		// Get the request body
-		const body = await request.arrayBuffer();
-
 		const response = await fetch(backendUrl, {
 			method: 'POST',
 			headers,
-			body
+			// Streamed, not buffered. `arrayBuffer()` held the whole upload in this
+			// process before forwarding a byte of it, which is survivable for one book
+			// and not for a zipped Calibre library. `duplex: 'half'` is required by the
+			// fetch spec whenever the body is a stream.
+			...bodyOf(request)
 		});
 
 		return handleResponse(response);
@@ -100,13 +115,10 @@ export const PATCH: RequestHandler = async ({ params, locals, fetch, request, ur
 		const backendUrl = `${BACKEND_API_URL}/${path}${queryString}`;
 		const headers = prepareRequest(locals, request);
 
-		// Get the request body
-		const body = await request.arrayBuffer();
-
 		const response = await fetch(backendUrl, {
 			method: 'PATCH',
 			headers,
-			body
+			...bodyOf(request)
 		});
 
 		return handleResponse(response);