fix(amazon): _record_to_status read 'filename' not 'original_filename'

The engine worker stores the encoded filename under the key 'filename'
(see worker.py dispatch). _record_to_status was reading 'original_filename',
which always returns "" — so every DownloadStatus emitted by
get_all_downloads/get_download_status had an empty filename string.

The download monitor builds lookup keys as
_make_context_key(download.username, download.filename). With filename=""
the key was always "amazon::" which never matched the task's
"amazon::B0B1234||Artist - Title" key. Monitor never detected Amazon
download completions, so tasks sat stuck at Downloading 0% forever even
though the files had actually downloaded.

Also fixes tests that had the same wrong key.

core/amazon_download_client.py

@@ -431,7 +431,7 @@ class AmazonDownloadClient(DownloadSourcePlugin):
     def _record_to_status(download_id: str, rec: Dict[str, Any]) -> DownloadStatus:
         return DownloadStatus(
             id=download_id,
-            filename=str(rec.get("original_filename", "")),
+            filename=str(rec.get("filename", "")),
             username="amazon",
             state=str(rec.get("state", "queued")),
             progress=float(rec.get("progress", 0.0)),

tests/tools/test_amazon_download_client.py

@@ -325,7 +325,7 @@ class TestUniquePath:
 class TestRecordToStatus:
     def test_fields_mapped(self):
         rec = {
-            "original_filename": "B1||Artist - Title",
+            "filename": "B1||Artist - Title",
             "state": "downloading",
             "progress": 0.5,
             "size": 10_000_000,
@@ -718,7 +718,7 @@ class TestStatusInterface:
         engine = MagicMock()
         engine.get_all_records.return_value = {
             "dl-001": {
-                "original_filename": "B1||A - T",
+                "filename": "B1||A - T",
                 "state": "complete",
                 "progress": 1.0,
                 "size": 5_000_000,