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/app/validate.go

@@ -24,6 +24,17 @@ func invalid(format string, args ...any) error {
 	return &ValidationError{msg: fmt.Sprintf(format, args...)}
 }
 
+// foldLogin is a new application's login as it is stored: in lower case
+// (audit 2026-10-12, H4; owner: fold, do not refuse). Postfix compares the
+// SASL login with the sender map's without regard to case, so two logins
+// that differ only in case would be one sender to Postfix and two
+// applications to the panel. The store refuses such a pair as well
+// (idx_applications_login_nocase); a login imported from another instance is
+// not folded, since its SASL account and its client use it as it is.
+func foldLogin(login string) string {
+	return strings.ToLower(login)
+}
+
 // validateLogin enforces a strict server-side whitelist for the SASL login
 // (security.md). It intentionally excludes '@': the login is stored in sasldb2,
 // where '@' separates the user from the realm, so allowing it would change the