feat: implement RBAC permissions system

Add role-based access control with hierarchy: super_admin > team_admin >
engineer > viewer. Adds is_super_admin boolean to User model (migration 010),
centralized backend permissions module, frontend usePermissions hook, and
UI enforcement (conditional Create/Edit buttons, editor redirect for viewers,
role badge in header). All endpoint admin checks updated from role=="admin"
to is_super_admin.

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

backend/app/schemas/tree.py

@@ -69,6 +69,7 @@ class TreeListResponse(BaseModel):
     category_info: Optional[CategoryInfo] = None
     tags: list[str] = []  # List of tag names
     author_id: Optional[UUID] = None
+    team_id: Optional[UUID] = None
     is_active: bool
     is_public: bool
     is_default: bool