feat: add cdr processing worker and dlq routing

tests/unit/test_dlq.py

@@ -0,0 +1,36 @@
+from core.domain.errors import (
+    CDRProcessingError,
+    FailureReason,
+)
+from core.worker.dlq import build_dlq_envelope
+
+
+def test_build_dlq_envelope() -> None:
+    original = {
+        "raw_id": "raw-invalid-001",
+        "rat": 99,
+    }
+
+    error = CDRProcessingError(
+        reason=FailureReason.INVALID_RAT_VALUE,
+        detail="Unsupported RAT value: 99.",
+        failed_field="rat",
+    )
+
+    envelope = build_dlq_envelope(
+        original_payload=original,
+        error=error,
+        worker_instance="worker-test",
+    )
+
+    assert envelope["original_payload"] == original
+
+    assert (
+        envelope["failure_reason"]
+        == "invalid_rat_value"
+    )
+
+    assert envelope["failed_field"] == "rat"
+    assert envelope["worker_instance"] == "worker-test"
+
+    assert envelope["failed_at"].endswith("Z")