feat: add jobs worker and webhook core

contracts/webhook/job-webhook-v1.json

@@ -0,0 +1,18 @@
+{
+  "version": 1,
+  "maximumAttempts": 3,
+  "contentType": "application/json",
+  "userAgent": "zhinian-aigc-webhook/1.0",
+  "signatureHeader": "X-Zhinian-Signature",
+  "secret": "webhook-secret",
+  "job": {
+    "id": "job-1",
+    "status": "failed",
+    "capability": "image.generate",
+    "outputAssetIds": [],
+    "error": { "message": "provider unavailable", "retryable": true },
+    "updatedAt": "2026-08-13T08:00:00.000Z"
+  },
+  "body": "{\"jobId\":\"job-1\",\"status\":\"failed\",\"capability\":\"image.generate\",\"outputAssetIds\":[],\"error\":{\"message\":\"provider unavailable\",\"retryable\":true},\"updatedAt\":\"2026-08-13T08:00:00.000Z\"}",
+  "signature": "sha256=32b8fd385692f6f549c0b1e1ffc9dce404c7a3a9a0aa19ca45225ad257d94411"
+}