Add is_default flag for system trees

- Add is_default column to trees table
- Default trees have no author and are visible to all users
- Only admins can create default trees
- Update seed script to mark seeded trees as default
- Update seed script to use CLI auth instead of creating seed user

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/schemas/tree.py

@@ -12,6 +12,7 @@ class TreeBase(BaseModel):
 
 class TreeCreate(TreeBase):
     tree_structure: dict[str, Any] = Field(..., description="The decision tree structure in JSON format")
+    is_default: bool = Field(False, description="Mark as a default/system tree (admin only)")
 
 
 class TreeUpdate(BaseModel):
@@ -28,6 +29,7 @@ class TreeResponse(TreeBase):
     author_id: Optional[UUID] = None
     team_id: Optional[UUID] = None
     is_active: bool
+    is_default: bool
     version: int
     created_at: datetime
     updated_at: datetime
@@ -43,6 +45,7 @@ class TreeListResponse(BaseModel):
     description: Optional[str] = None
     category: Optional[str] = None
     is_active: bool
+    is_default: bool
     version: int
     usage_count: int
     created_at: datetime