Fix: include last-modified in doc etag (#13044)

src/documents/conditionals.py

@@ -70,13 +70,13 @@ def suggestions_last_modified(request, pk: int) -> datetime | None:
 
 def metadata_etag(request, pk: int) -> str | None:
     """
-    Metadata is extracted from the original file, so use its checksum as the
-    ETag
+    Metadata responses include metadata as well as document fields, so include
+    the modification time with the checksum so metadata-only changes invalidate cache.
     """
     doc = resolve_effective_document_by_pk(pk, request).document
     if doc is None:
         return None
-    return doc.checksum
+    return f"{doc.checksum}:{doc.modified.isoformat()}"
 
 
 def metadata_last_modified(request, pk: int) -> datetime | None: