fix(files): verify acquired source identity and index import lookups

Release v0.1.27. Coordinated integrity review: GovOPlaN/govoplan-core#298.

src/govoplan_files/backend/storage/common.py

@@ -10,6 +10,10 @@ class FileStorageError(RuntimeError):
     pass
 
 
+class FileSourceConflict(FileStorageError):
+    """A source cannot be selected or revision-bound without ambiguity."""
+
+
 @dataclass(slots=True)
 class UploadedStoredFile:
     asset: FileAsset