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>

backend/app/services/patreon_client.py

@@ -405,21 +405,19 @@ class PatreonClient:
                     return candidate
         return None
 
-    def _media_item(
-        self, attrs: dict, kind: str, post_id: str, *, require_file_name: bool
-    ) -> MediaItem:
+    def _media_item(self, attrs: dict, kind: str, post_id: str) -> MediaItem:
         url = self._media_url(attrs)
         if not url:
             raise PatreonDriftError(
                 f"Patreon media (post {post_id}, kind={kind}) had no resolvable URL "
                 f"(no download_url / image_urls)"
             )
+        # file_name is OPTIONAL: Patreon legitimately serves some gallery images
+        # without it (operator-flagged 2026-06-07, BlenderKnight post 73665615),
+        # and the URL basename is a fine fallback — the same thing gallery-dl
+        # uses. A genuine schema change shows up as no URL (above) or a media id
+        # absent from `included` (caller), not a missing name.
         file_name = attrs.get("file_name")
-        if require_file_name and not (isinstance(file_name, str) and file_name):
-            raise PatreonDriftError(
-                f"Patreon media resource (post {post_id}, kind={kind}) missing "
-                f"'file_name'"
-            )
         filename = file_name if isinstance(file_name, str) and file_name else _basename_from_url(url)
         return MediaItem(
             url=url,
@@ -452,11 +450,7 @@ class PatreonClient:
                         f"Patreon post {post_id} references media {mid} "
                         f"({rel_name}) not present in 'included'"
                     )
-                items.append(
-                    self._media_item(
-                        media_attrs, kind, post_id, require_file_name=True
-                    )
-                )
+                items.append(self._media_item(media_attrs, kind, post_id))
 
         # 1. gallery images
         _resolve_rel("images", "images")