fix(family-sharing): prevent silent data loss when rehoming or removing users (#1896)

* fix(family-sharing): prevent silent data loss when rehoming or removing users

Fixes #1689.

Two destructive paths could strand a pre-existing user's family and accounts:

1. Invitation#accept_for unconditionally overwrote user.family_id, orphaning
   the prior family + its accounts with no user able to reach them.
2. Settings::ProfilesController#destroy then called @user.destroy when an admin
   removed the rehomed member, destroying the only login path back to the
   now-orphaned data.

Add hard-block guards on both paths. accept_for refuses when the invitee
already belongs to a family with accounts; ProfilesController#destroy refuses
when the member owns accounts in another family (legacy state from the old
flow). InvitationsController#create surfaces a specific, actionable flash so
the admin understands why the auto-accept was refused.

No automatic recovery of already-orphaned data — that needs a separate
one-shot script per dosubot's analysis on the issue.

* fix(family-sharing): scope invite orphan-guard to invitee-owned accounts (#1896 review)

Codex flagged (P1) and the maintainer review independently raised that
would_orphan_existing_family? keyed off user.family.accounts.exists? —
any account in the invitee's current family — which wrongly blocked a
non-owner member from leaving a multi-user household.

Rename to would_orphan_owned_accounts? and key off
user.owned_accounts.where.not(family_id: family_id), making the invite
guard symmetric with the destroy-path guard in
Settings::ProfilesController. A member who owns no accounts now orphans
nothing by moving and is free to accept the invitation; an owner is
still blocked.

Add a regression test for the non-owner case and update the existing
tests to give the invitee explicit account ownership.

* Remove extra comments per project conventions

---------

Co-authored-by: Juan José Mata <jjmata@jjmata.com>

test/models/invitation_test.rb

@@ -122,6 +122,65 @@ class InvitationTest < ActiveSupport::TestCase
     assert_includes invitation.errors[:email], "has already been invited to this family"
   end
 
+  test "accept_for refuses when invitee owns accounts that would be orphaned" do
+    owner = users(:empty)
+    owner_family = families(:empty)
+    owner.update_columns(family_id: owner_family.id, role: "admin")
+    account = owner_family.accounts.create!(
+      name: "Prior savings", balance: 100, currency: "USD",
+      accountable: Depository.new
+    )
+    account.update_columns(owner_id: owner.id)
+
+    invitation = @family.invitations.create!(email: owner.email, role: "member", inviter: @inviter)
+
+    result = invitation.accept_for(owner)
+
+    assert_not result, "accept_for must refuse to rehome a user away from accounts they own"
+    owner.reload
+    assert_equal owner_family.id, owner.family_id, "user.family_id must not be silently overwritten"
+    invitation.reload
+    assert_nil invitation.accepted_at, "invitation must remain pending so a new flow can recover"
+    assert owner_family.accounts.exists?, "original family's accounts must remain intact"
+  end
+
+  test "accept_for allows a member who owns no accounts to join another family" do
+    member = users(:empty)
+    other_owner = users(:sure_support_staff)
+    source_family = families(:empty)
+    member.update_columns(family_id: source_family.id, role: "member")
+    other_owner.update_columns(family_id: source_family.id, role: "admin")
+    account = source_family.accounts.create!(
+      name: "Shared savings", balance: 100, currency: "USD",
+      accountable: Depository.new
+    )
+    account.update_columns(owner_id: other_owner.id)
+
+    invitation = @family.invitations.create!(email: member.email, role: "member", inviter: @inviter)
+
+    result = invitation.accept_for(member)
+
+    assert result, "a non-owner member must be free to join another family"
+    member.reload
+    assert_equal @family.id, member.family_id
+  end
+
+  test "would_orphan_owned_accounts? is false when invitee owns no accounts" do
+    user = users(:empty)
+    user.update_columns(family_id: families(:empty).id, role: "admin")
+    invitation = @family.invitations.create!(email: user.email, role: "member", inviter: @inviter)
+
+    assert_not invitation.would_orphan_owned_accounts?(user)
+  end
+
+  test "would_orphan_owned_accounts? is false when same-family role change" do
+    user = users(:family_member)
+    user.update!(family_id: @family.id, role: "member")
+    invitation = @family.invitations.create!(email: user.email, role: "admin", inviter: @inviter)
+
+    assert_not invitation.would_orphan_owned_accounts?(user)
+  end
+
   test "accept_for applies guest role defaults" do
     user = users(:family_member)
     user.update!(