Docs: permission hardening Role and member user link

Role: Ash policies (HasPermission); read for all, create/update/destroy admin only.
User–member link: only admins may set :user on Member create/update (ForbidMemberUserLinkUnlessAdmin).

docs/roles-and-permissions-architecture.md

@@ -1025,17 +1025,16 @@ defmodule Mv.Membership.Member do
       authorize_if expr(id == ^actor(:member_id))
     end
     
-    # 2. GENERAL: Check permissions from role
-    # - :own_data → can UPDATE linked member (scope :linked via HasPermission)
-    # - :read_only → can READ all members (scope :all), no update permission
-    # - :normal_user → can CRUD all members (scope :all)
-    # - :admin → can CRUD all members (scope :all)
+    # 2. GENERAL: Forbid user link unless admin; then check permissions from role
+    # ForbidMemberUserLinkUnlessAdmin: only admins may pass :user on create/update (no-op for read/destroy)
+    # HasPermission: :own_data → update linked; :read_only → no update; :normal_user/admin → update all
     policy action_type([:read, :create, :update, :destroy]) do
-      description "Check permissions from user's role"
+      description "Check permissions and forbid user link unless admin"
+      forbid_if Mv.Authorization.Checks.ForbidMemberUserLinkUnlessAdmin
       authorize_if Mv.Authorization.Checks.HasPermission
     end
     
-    # 4. DEFAULT: Ash implicitly forbids if no policy authorizes (fail-closed)
+    # 3. DEFAULT: Ash implicitly forbids if no policy authorizes (fail-closed)
   end
   
   # Custom validation for email editing (see Special Cases section)
@@ -1054,6 +1053,8 @@ end
 - **READ list queries**: No record at strict_check time → bypass with `expr(id == ^actor(:member_id))` needed for auto_filter ✅
 - **UPDATE operations**: Changeset contains record → HasPermission evaluates `scope :linked` correctly ✅
 
+**User–member link:** Only admins may set or change the `:user` argument on create_member or update_member (see [User-Member Linking](#user-member-linking)). Non-admins can create/update members without passing `:user`.
+
 **Permission Matrix:**
 
 | Action | Mitglied | Vorstand | Kassenwart | Buchhaltung | Admin |
@@ -1148,23 +1149,20 @@ end
 
 **Location:** `lib/mv/authorization/role.ex`
 
-**Special Protection:** System roles cannot be deleted.
+**Defense-in-depth:** The Role resource uses `authorizers: [Ash.Policy.Authorizer]` and policies with `Mv.Authorization.Checks.HasPermission`. **Read** is allowed for all permission sets (own_data, read_only, normal_user, admin) via `perm("Role", :read, :all)` in PermissionSets; reading roles is not a security concern. **Create, update, and destroy** are allowed only for admin (admin has full Role CRUD in PermissionSets). Seeds and bootstrap use `authorize?: false` where necessary.
+
+**Special Protection:** System roles cannot be deleted (validation on destroy).
 
 ```elixir
 defmodule Mv.Authorization.Role do
-  use Ash.Resource, ...
+  use Ash.Resource,
+    authorizers: [Ash.Policy.Authorizer]
   
   policies do
-    # Only admin can manage roles
     policy action_type([:read, :create, :update, :destroy]) do
-      description "Check permissions from user's role"
+      description "Check permissions from user's role (read all, create/update/destroy admin only)"
       authorize_if Mv.Authorization.Checks.HasPermission
     end
-    
-    # DEFAULT: Forbid
-    policy action_type([:read, :create, :update, :destroy]) do
-      forbid_if always()
-    end
   end
   
   # Prevent deletion of system roles
@@ -1201,7 +1199,7 @@ end
 
 | Action | Mitglied | Vorstand | Kassenwart | Buchhaltung | Admin |
 |--------|----------|----------|------------|-------------|-------|
-| Read | ❌ | ❌ | ❌ | ❌ | ✅ |
+| Read | ✅ | ✅ | ✅ | ✅ | ✅ |
 | Create | ❌ | ❌ | ❌ | ❌ | ✅ |
 | Update | ❌ | ❌ | ❌ | ❌ | ✅ |
 | Destroy* | ❌ | ❌ | ❌ | ❌ | ✅ |
@@ -2045,7 +2043,10 @@ Users and Members are separate entities that can be linked. Special rules:
 - A user cannot link themselves to an existing member
 - A user CAN create a new member and be directly linked to it (self-service)
 
-**Enforcement:** The User resource restricts the `update_user` action (which accepts the `member` argument for link/unlink) to admins only via `Mv.Authorization.Checks.ActorIsAdmin`. The UserLive.Form shows the Member-Linking UI and runs member link/unlink on save only when the current user is admin; non-admins use the `:update` action (email only) for profile edit.
+**Enforcement:**
+
+- **User side:** The User resource restricts the `update_user` action (which accepts the `member` argument for link/unlink) to admins only via `Mv.Authorization.Checks.ActorIsAdmin`. The UserLive.Form shows the Member-Linking UI and runs member link/unlink on save only when the current user is admin; non-admins use the `:update` action (email only) for profile edit.
+- **Member side:** Only admins may set or change the user–member link on **Member** create or update. When creating or updating a member, the `:user` argument (which links the member to a user account) is forbidden for non-admins. This is enforced by `Mv.Authorization.Checks.ForbidMemberUserLinkUnlessAdmin` in the Member resource policies (`forbid_if` before `authorize_if HasPermission`). Non-admins (e.g. normal_user / Kassenwart) can still create and update members as long as they do not pass the `:user` argument.
 
 ### Approach: Separate Ash Actions