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>

frontend/src/types/tree.ts

@@ -86,6 +86,7 @@ export interface TreeListItem {
   category_info: CategoryInfo | null
   tags: string[]
   author_id: string | null
+  team_id: string | null
   is_active: boolean
   is_public: boolean
   is_default: boolean

frontend/src/types/user.ts

@@ -1,10 +1,11 @@
-export type UserRole = 'admin' | 'engineer' | 'viewer'
+export type UserRole = 'engineer' | 'viewer'
 
 export interface User {
   id: string
   email: string
   name: string
   role: UserRole
+  is_super_admin: boolean
   is_team_admin: boolean
   team_id: string | null
   created_at: string