Make the download deadline an idle deadline and cancel failed bodies (closes #24)

downloadTimeoutMs now aborts a file or thumbnail download only when no
bytes have arrived for that long (default 60 s, was a 600 s cap on the
whole transfer), so a slow download that keeps making progress completes.
The abort reason is still a TimeoutError, so retry classification is
unchanged. streamDecrypt cancels the response body when decryption or
the write fails, so a failed file no longer holds its connection.

Model: opus-5-5

test/download/download.test.ts

@@ -1337,6 +1337,41 @@ describe("download retries: corruption is not retried", () => {
 
         expect(requests()).toBe(1);
     });
+
+    it("cancels the response body when decryption fails", async () => {
+        // A backup run carries on past a failed file, so a body left open on
+        // failure would hold its connection until garbage collection, once
+        // per failed file. This body delivers a corrupt chunk and then stays
+        // open, so only a cancel from the downloader can close it.
+        const corrupted = Uint8Array.from(multiChunk.body);
+        corrupted[10] ^= 0xff;
+        let cancelled = false;
+        const fetch = (async () =>
+            new Response(
+                new ReadableStream<Uint8Array>({
+                    start(controller) {
+                        controller.enqueue(corrupted);
+                    },
+                    cancel() {
+                        cancelled = true;
+                    },
+                }),
+                { status: 200 },
+            )) as typeof globalThis.fetch;
+        const api = new ApiClient({ fetch, retry: { ...noWait, attempts: 1 } });
+        const file = buildMockEnteFile(
+            multiChunkKey,
+            multiChunk.header,
+            multiChunk.header,
+        );
+        const outPath = join(mkdtempSync(join(testDir, "cancel-")), "c.bin");
+
+        await expect(downloadFile(api, file, outPath)).rejects.toThrow(
+            /authentication failed/i,
+        );
+
+        expect(cancelled).toBe(true);
+    });
 });
 
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