Bound and deduplicate server image rendering per account

src/rendering/queue.test.ts

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+import { expect, test } from 'bun:test';
+import { createRenderQueue, RenderBusyError } from './queue';
+
+const result = new Uint8Array([1, 2, 3]);
+test('render queue bounds global concurrency and shares only the same owner and content', async () => {
+  const queue = createRenderQueue({ concurrency: 2, maxPending: 4, maxPerUser: 2 });
+  let active = 0, peak = 0, calls = 0;
+  const run = async () => { calls++; active++; peak = Math.max(peak, active); await Bun.sleep(5); active--; return result; };
+  const first = queue('alice', 'one', run);
+  expect(queue('alice', 'one', run)).toBe(first);
+  const all = [first, queue('alice', 'two', run), queue('bob', 'one', run), queue('carol', 'one', run)];
+  await Promise.all(all); expect(calls).toBe(4); expect(peak).toBe(2);
+  expect(await queue('alice', 'one', run)).toBe(result); expect(calls).toBe(5);
+});
+
+test('render admission enforces per-user and global capacity and recovers after failures', async () => {
+  const queue = createRenderQueue({ concurrency: 1, maxPending: 2, maxPerUser: 1 });
+  let release!: () => void;
+  const first = queue('alice', 'a', async () => { await new Promise<void>(resolve => { release = resolve; }); throw new Error('render failed'); });
+  const failure = first.catch(error => error);
+  await expect(queue('alice', 'b', async () => result)).rejects.toBeInstanceOf(RenderBusyError);
+  const second = queue('bob', 'b', async () => result);
+  await expect(queue('carol', 'c', async () => result)).rejects.toBeInstanceOf(RenderBusyError);
+  while (!release) await Bun.sleep(1);
+  release(); expect((await failure).message).toBe('render failed'); await second;
+  expect(await queue('alice', 'a', async () => result)).toBe(result);
+});