findings of the reviews of audit stage 2

Technical review (Opus) and security review (Fable) of a59e3d4..cb5a4a2;
reports in the session's scratchpad. Nothing critical, high or medium; no
outright bug. Fixed:

- F2, F3 (0008 backfill, before it runs on the live database): rows are
  filled by the rule new rows follow - the application's domain, then the
  name's - and only by a domain and application that existed when the row
  was written (created_at); the inbound log likewise. Test: the migration's
  UPDATEs re-run on prepared rows (before the domain, after, null sender,
  application gone, a stranger's name). Dry run on a copy of the live
  database: 68/68 and 10/10 rows filled, none disagreeing with its login's
  domain, integrity ok.
- V2, F4: an application's statistics and level-2 count are its login's
  rows of its domain (appByLogin), and StatsWindow starts no earlier than
  the entity's creation; the message page's "may send as" only for a login
  still of the row's domain. Test: a login taken again in another domain
  starts at zero.
- F7: MAIL FROM:<> is held to the domain limit of its login
  (ApplicationByLogin fills DomainName).
- V3: an imported DKIM selector is held to DNS labels (validate.Selector).
- F5, F6: an imported auto limit's numbers are dropped and recalculated
  right after the import; its mode is stored trimmed.
- V6: the store refuses a role that is neither global nor domain.
- F9: SaveUser keeps the stored hash when no new password is given (the
  form no longer writes back the hash it read). F10: a reach naming a
  domain deleted meanwhile is ErrNoSuchDomain, answered on the form.
- V4, F12: from-lines-other.txt - lines naming another sender ({other}) in
  a quoted string, a quoted pair, capitals, a comment, an encoded word, a
  quoted local part; the unit test wants each refused, the e2e
  (lines_naming_another_sender_never_signed_for_it) wants each refused and
  none signed for it. The passed list gained a.{DOMAIN}, a comment after
  the angle address, a quoted local part, and "b@ {other}" - passed by
  design (security.md), and the e2e shows it signed for the own domain.
- F13: e2e login_in_other_capitals_does_not_authenticate - AUTH with the
  login upper-cased gets 535.

Not changed, written down: V1/F1 - L8 is half closed (refused or passed
still tells an application which domains are hosted here); security.md's
accepted risks and the CHANGELOG say so, and the owner is asked whether to
refuse every foreign address in a display name (changes the 2.4.0 rule).
F11 - deleting a domain rewrites its rows' domain_id (seconds at 1e5 rows),
accepted at this size. F8 (test cannot tell write-first from read-first;
the reviewer verified it with two stores, 30/30), V5, V7, P2: noted.

Evidence on the test server for this tree: go vet clean, go test ./... 23
packages ok, make e2e exit 0 with the three new cases.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/store/users_tx_test.go

@@ -149,3 +149,46 @@ func TestCreateGlobalUserOnlyOnce(t *testing.T) {
 		t.Fatalf("%d setups succeeded, %d global users; want 1 and 1", ok, n)
 	}
 }
+
+// The store refuses a role that is neither global nor domain.
+func TestTheStoreKnowsOnlyTwoRoles(t *testing.T) {
+	st := openTestStore(t)
+	if _, err := st.AddUser(UserChange{Username: "x", PasswordHash: "h", Role: "admin"}); err == nil {
+		t.Fatal("AddUser took the role admin")
+	}
+	id, err := st.CreateUser("y", "h", RoleGlobal, Reach{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := st.SaveUser(UserChange{ID: id, Username: "y", PasswordHash: "h", Role: ""}); err == nil {
+		t.Fatal("SaveUser took an empty role")
+	}
+}
+
+// A save without a new password keeps the stored hash - one the user may have
+// changed on Account since the form was opened - and a reach naming a domain
+// deleted meanwhile is ErrNoSuchDomain (review of audit stage 2).
+func TestSaveUserKeepsTheHashAndNamesAMissingDomain(t *testing.T) {
+	st := openTestStore(t)
+	d := addTestDomain(t, st, "example.com")
+	if _, err := st.CreateUser("root", "hash", RoleGlobal, Reach{}); err != nil {
+		t.Fatal(err)
+	}
+	id, err := st.CreateUser("shop", "old", RoleDomain, Reach{DomainIDs: []int64{d.ID}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := st.UpdateUser(id, "shop", "changed-on-account", ""); err != nil {
+		t.Fatal(err)
+	}
+	if err := st.SaveUser(UserChange{ID: id, Username: "shop", Role: RoleDomain, Reach: Reach{DomainIDs: []int64{d.ID}}}); err != nil {
+		t.Fatal(err)
+	}
+	u, _ := st.GetUser(id)
+	if u.PasswordHash != "changed-on-account" {
+		t.Fatalf("the hash became %q", u.PasswordHash)
+	}
+	if err := st.SaveUser(UserChange{ID: id, Username: "shop", Role: RoleDomain, Reach: Reach{DomainIDs: []int64{9999}}}); !errors.Is(err, ErrNoSuchDomain) {
+		t.Fatalf("a reach to a deleted domain = %v, want ErrNoSuchDomain", err)
+	}
+}