Security: Fix critical deny-filter bug and improve authorization

CRITICAL FIX: Deny-filter was allowing all records instead of denying
Fix: User validation in Member now uses actor from changeset.context

lib/membership/member.ex

@@ -300,12 +300,12 @@ defmodule Mv.Membership.Member do
   # Authorization Policies
   # Order matters: Most specific policies first, then general permission check
   policies do
-    # SYSTEM OPERATIONS: Allow CRUD operations without actor
+    # SYSTEM OPERATIONS: Allow CRUD operations without actor (TEST ENVIRONMENT ONLY)
     # In test: All operations allowed (for test fixtures)
-    # In production: Only :create and :read allowed (enforced by NoActor.check)
-    # :read is needed for internal Ash lookups (e.g., relationship validation during user creation).
+    # In production/dev: ALL operations denied without actor (fail-closed for security)
+    # NoActor.check uses compile-time environment detection to prevent security issues
     bypass action_type([:create, :read, :update, :destroy]) do
-      description "Allow system operations without actor (seeds, tests, internal lookups)"
+      description "Allow system operations without actor (test environment only)"
       authorize_if Mv.Authorization.Checks.NoActor
     end
 
@@ -399,8 +399,13 @@ defmodule Mv.Membership.Member do
         user_id = user_arg[:id]
         current_member_id = changeset.data.id
 
+        # Get actor from changeset context for authorization
+        # If no actor is present, this will fail in production (fail-closed)
+        actor = Map.get(changeset.context || %{}, :actor)
+
         # Check the current state of the user in the database
-        case Ash.get(Mv.Accounts.User, user_id) do
+        # Pass actor to ensure proper authorization (User might have policies in future)
+        case Ash.get(Mv.Accounts.User, user_id, actor: actor) do
           # User is free to be linked
           {:ok, %{member_id: nil}} ->
             :ok