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/src/image-derivative-worker.js

@@ -0,0 +1,26 @@
+export async function runImageDerivativeJob({ job, loadSource, processor }) {
+  if (!job || job.type !== 'IMAGE_DERIVATIVES') {
+    throw new Error('Unsupported image derivative job.');
+  }
+
+  const processingJob = { ...job, status: 'PROCESSING', error: null };
+
+  try {
+    const source = await loadSource(job.sourceKey);
+    const derivatives = await processor.process(source);
+
+    return {
+      ...processingJob,
+      status: 'COMPLETED',
+      derivatives,
+      completedAt: new Date(),
+    };
+  } catch (error) {
+    return {
+      ...processingJob,
+      status: 'FAILED',
+      error: error instanceof Error ? error.message : 'Image derivative processing failed.',
+      failedAt: new Date(),
+    };
+  }
+}