fix(patreon): a missing media file_name is a URL-basename fallback, not API drift

The native client treated a gallery image without `file_name` as schema drift
and raised "Patreon API changed — ingester needs update", failing the whole walk
(operator-flagged 2026-06-07: BlenderKnight post 73665615, kind=images). But the
resource had a valid URL, and the code already derives a filename from the URL
basename right below the raise — the same fallback gallery-dl uses. Patreon
legitimately serves some images without file_name, so this isn't drift.

Drop the require_file_name gate from _media_item: file_name is now optional for
every kind (images/attachments/postfile), falling back to the URL basename.
Genuine drift still raises — no resolvable URL, or a media id referenced by a
relationship but absent from `included`. Test updated to assert the fallback.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_patreon_client.py

@@ -159,9 +159,10 @@ def test_non_list_data_raises_drift(client):
         client._validate_response({"data": {"id": "1"}})
 
 
-def test_media_missing_file_name_raises_drift(client):
-    # A media resource referenced by a gallery relationship but lacking
-    # file_name is drift from the contract.
+def test_media_missing_file_name_falls_back_to_url_basename(client):
+    # A gallery image with a valid URL but no file_name is NOT drift — Patreon
+    # serves these (operator-flagged 2026-06-07, BlenderKnight post 73665615);
+    # fall back to the URL basename like gallery-dl does.
     post = {
         "id": "2000",
         "type": "post",
@@ -175,8 +176,10 @@ def test_media_missing_file_name_raises_drift(client):
             + "/x.jpg"
         }
     }
-    with pytest.raises(PatreonDriftError):
-        client.extract_media(post, index)
+    items = client.extract_media(post, index)
+    assert len(items) == 1
+    assert items[0].filename == "x.jpg"
+    assert items[0].kind == "images"
 
 
 def test_media_referenced_but_absent_raises_drift(client):