fix(admin): allow owner and admin account roles in user creation and role management

Four places were hardcoded to engineer|viewer only:
- AccountRoleUpdate schema (user.py) — blocked PUT /admin/users/{id}/account-role at the API level
- AdminUserCreate schema (admin.py) — blocked creating users with owner/admin role
- AccountDetailPage role dropdowns (create form + inline member role changer)
- AccountsPage create user role dropdown

Now all four accept the full set: owner, admin, engineer, viewer.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/schemas/user.py

@@ -68,4 +68,4 @@ class RoleUpdate(BaseModel):
 
 
 class AccountRoleUpdate(BaseModel):
-    account_role: str = Field(..., pattern="^(engineer|viewer)$")
+    account_role: str = Field(..., pattern="^(owner|admin|engineer|viewer)$")