BE-PR-009: restrict all story/* task types to Proposal Accept workflow

- Expand RESTRICTED_TYPE_SUBTYPES to include story/feature, story/improvement,
  story/refactor, and story/None (all story subtypes)
- Add FULLY_RESTRICTED_TYPES fast-path set for entire-type blocking
- Update _validate_task_type_subtype to block all story types via general
  create endpoint with clear error message directing to Proposal Accept
- Add type/subtype validation to PATCH /tasks/{id} to prevent changing
  existing tasks to story/* type via update
- Internal Proposal Accept flow unaffected (creates tasks directly via ORM)

app/api/routers/tasks.py

@@ -66,15 +66,22 @@ TASK_SUBTYPE_MAP = {
 ALLOWED_TASK_TYPES = set(TASK_SUBTYPE_MAP.keys())
 
 
-"""P9.6 — type+subtype combos that may NOT be created via general create endpoints.
-  feature story → must come from propose accept
-  release maintenance → must come from controlled milestone/release flow
+"""P9.6 / BE-PR-009 — type+subtype combos that may NOT be created via general
+  endpoints.  All story/* subtypes are restricted; they must come from Proposal
+  Accept.  maintenance/release must come from the milestone release flow.
 """
 RESTRICTED_TYPE_SUBTYPES = {
     ("story", "feature"),
+    ("story", "improvement"),
+    ("story", "refactor"),
+    ("story", None),           # story with no subtype is also blocked
     ("maintenance", "release"),
 }
 
+# Convenience set: task types whose *entire* type is restricted regardless of subtype.
+# Used for a fast-path check so we don't need to enumerate every subtype.
+FULLY_RESTRICTED_TYPES = {"story"}
+
 
 def _validate_task_type_subtype(task_type: str | None, task_subtype: str | None, *, allow_restricted: bool = False):
     if task_type is None:
@@ -84,13 +91,23 @@ def _validate_task_type_subtype(task_type: str | None, task_subtype: str | None,
     allowed = TASK_SUBTYPE_MAP.get(task_type, set())
     if task_subtype and task_subtype not in allowed:
         raise HTTPException(status_code=400, detail=f'Invalid task_subtype for {task_type}: {task_subtype}')
-    # P9.6: block restricted combos unless explicitly allowed (e.g. propose accept, internal create)
-    if not allow_restricted and (task_type, task_subtype) in RESTRICTED_TYPE_SUBTYPES:
-        raise HTTPException(
-            status_code=400,
-            detail=f"Cannot create {task_type}/{task_subtype} task via general create. "
-                   f"Use the appropriate workflow (propose accept / milestone release setup)."
-        )
+    # P9.6 / BE-PR-009: block restricted combos unless explicitly allowed
+    # (e.g. Proposal Accept, internal create)
+    if not allow_restricted:
+        # Fast-path: entire type is restricted (all story/* combos)
+        if task_type in FULLY_RESTRICTED_TYPES:
+            raise HTTPException(
+                status_code=400,
+                detail=f"Cannot create '{task_type}' tasks via general endpoints. "
+                       f"Use the Proposal Accept workflow instead.",
+            )
+        # Specific type+subtype combos (e.g. maintenance/release)
+        if (task_type, task_subtype) in RESTRICTED_TYPE_SUBTYPES:
+            raise HTTPException(
+                status_code=400,
+                detail=f"Cannot create {task_type}/{task_subtype} task via general create. "
+                       f"Use the appropriate workflow (Proposal Accept / milestone release setup).",
+            )
 
 
 def _notify_user(db, user_id, ntype, title, message=None, entity_type=None, entity_id=None):
@@ -383,6 +400,16 @@ def update_task(task_id: str, task_update: schemas.TaskUpdate, db: Session = Dep
                     detail="Only the current assignee or an admin can edit this task",
                 )
 
+    # BE-PR-009: prevent changing task_type to a restricted type via PATCH
+    new_task_type = update_data.get("task_type")
+    new_task_subtype = update_data.get("task_subtype", task.task_subtype)
+    if new_task_type is not None:
+        _validate_task_type_subtype(new_task_type, new_task_subtype)
+    elif "task_subtype" in update_data:
+        # subtype changed but type unchanged — validate the combo
+        current_type = task.task_type.value if hasattr(task.task_type, "value") else (task.task_type or "issue")
+        _validate_task_type_subtype(current_type, new_task_subtype)
+
     # Legacy general permission check (covers project membership etc.)
     ensure_can_edit_task(db, current_user.id, task)
     if "status" in update_data: