feat: add image derivative worker lifecycle

Add worker-owned image derivative job execution with explicit PROCESSING, COMPLETED, and FAILED outcomes, plus tests and documentation. Verification: npm test passed 70 tests, worker tests passed 2 tests, and integration passed 14 tests. No build script is configured.

apps/worker/tests/image-derivative-worker.test.js

@@ -0,0 +1,40 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+
+import { runImageDerivativeJob } from '../src/image-derivative-worker.js';
+
+test('completes an image derivative job after loading and processing the source', async () => {
+  const calls = [];
+  const result = await runImageDerivativeJob({
+    job: { id: 'job-1', type: 'IMAGE_DERIVATIVES', sourceKey: 'source.jpg', status: 'PENDING' },
+    async loadSource(sourceKey) {
+      calls.push(`load:${sourceKey}`);
+      return { sourceKey, data: Buffer.from('image') };
+    },
+    processor: {
+      async process(source) {
+        calls.push(`process:${source.sourceKey}`);
+        return [{ name: 'thumbnail', key: 'source-thumbnail.webp' }];
+      },
+    },
+  });
+
+  assert.equal(result.status, 'COMPLETED');
+  assert.deepEqual(result.derivatives, [{ name: 'thumbnail', key: 'source-thumbnail.webp' }]);
+  assert.deepEqual(calls, ['load:source.jpg', 'process:source.jpg']);
+  assert.ok(result.completedAt instanceof Date);
+});
+
+test('records a failed image derivative job without throwing worker errors', async () => {
+  const result = await runImageDerivativeJob({
+    job: { id: 'job-2', type: 'IMAGE_DERIVATIVES', sourceKey: 'missing.jpg', status: 'PENDING' },
+    async loadSource() {
+      throw new Error('source unavailable');
+    },
+    processor: { async process() { return []; } },
+  });
+
+  assert.equal(result.status, 'FAILED');
+  assert.equal(result.error, 'source unavailable');
+  assert.ok(result.failedAt instanceof Date);
+});