feat(backend): add editor context, draft flow bootstrap, and draft summary endpoints

add backend endpoints for editor context and draft summaries

ensure draft flow bootstrap for editor-driven workflows
improve draft-aware initialization and summary reads for clients

backend/app/schemas/scheme_versions.py

@@ -1,5 +1,3 @@
-from typing import List
-
 from pydantic import BaseModel
 
 
@@ -17,7 +15,7 @@ class SchemeVersionListItem(BaseModel):
 
 
 class SchemeVersionListResponse(BaseModel):
-    items: List[SchemeVersionListItem]
+    items: list[SchemeVersionListItem]
     total: int
 
 
@@ -27,3 +25,13 @@ class SchemeVersionCreateResponse(BaseModel):
     version_number: int
     status: str
     normalized_storage_path: str
+
+
+class EnsureDraftResponse(BaseModel):
+    scheme_id: str
+    scheme_version_id: str
+    version_number: int
+    status: str
+    normalized_storage_path: str
+    created: bool
+    source_scheme_version_id: str | None = None