app: logins folded to lower case, unique without regard to case (audit H4, 0008)

Postfix's reject_sender_login_mismatch compares the SASL login with the
sender map without regard to case - seen on the e2e stand this time (a
one-off probe, not committed: map rewritten to "APP1", client authenticated
as "app1", RCPT accepted). The panel stored logins as typed under a BINARY
UNIQUE, so "Ops" and "ops" were two applications to the panel and one sender
to Postfix: a tenant could send with another tenant's envelope sender.

- app.foldLogin: Service.Create lower-cases the login (owner: fold, do not
  refuse); CreateWithSettings rolls back the stored (folded) login.
- ImportApplication keeps the login as written: folding it would rename the
  SASL account and break the imported application's working credentials;
  the index below refuses a case clash there too.
- Migration 0008 (DDL): CREATE UNIQUE INDEX idx_applications_login_nocase ON
  applications (login COLLATE NOCASE). The column's own BINARY UNIQUE stays
  (SQLite cannot change a collation without a table rebuild, which a
  migration transaction cannot do with foreign keys on). Checked on the live
  installation first: 4 logins, no case clash; "Quieter3C" keeps its case.
  0008 is not run anywhere yet; send_log.domain_id joins it later in stage 2.

Tests: "Ops" created as ops, SASL account and map binding in lower case,
OPS in another domain and an imported oPs refused; the store refuses ops
beside Ops; schema at user_version 8 with the index.

CHANGELOG [Unreleased], audit plan line ticked, roadmap 15/61.
Evidence on the test server for this tree (with the probe): go vet clean,
go test ./... 23 packages ok, make e2e exit 0.

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

internal/store/applications_test.go

@@ -38,6 +38,20 @@ func TestAddApplicationWildcard(t *testing.T) {
 	}
 }
 
+// Logins are unique without regard to case, whatever the caller passed
+// (idx_applications_login_nocase, migration 0008).
+func TestAddApplicationRefusesALoginDifferingOnlyInCase(t *testing.T) {
+	st := openTestStore(t)
+	a := addTestDomain(t, st, "a.example")
+	b := addTestDomain(t, st, "b.example")
+	if _, err := st.AddApplication(a.ID, "Ops", AddressModeWildcard, nil); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := st.AddApplication(b.ID, "ops", AddressModeWildcard, nil); !errors.Is(err, ErrLoginExists) {
+		t.Fatalf("ops beside Ops = %v, want ErrLoginExists", err)
+	}
+}
+
 func TestAddApplicationListStoresAddresses(t *testing.T) {
 	st := openTestStore(t)
 	d := addTestDomain(t, st, "example.com")