feat: redesign metadata as KV store, add custom properties and WOPI Discovery

Replace one-to-one FileMetadata table with flexible ObjectMetadata KV pairs,
add custom property definitions, WOPI Discovery auto-configuration, and
per-extension action URL support.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/fixtures/objects.py

@@ -92,9 +92,9 @@ class ObjectFactory:
             owner_id=owner_id,
             policy_id=policy_id,
             size=size,
+            mime_type=kwargs.get("mime_type"),
             source_name=kwargs.get("source_name", name),
             upload_session_id=kwargs.get("upload_session_id"),
-            file_metadata=kwargs.get("file_metadata"),
             password=kwargs.get("password"),
         )