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>

docs/audit-2026-10-12.md

@@ -642,7 +642,7 @@ has already run is never edited — the next file is added; the published
 
 ### Stage 2 — the tenant boundary and the registry (H4, M7, M9, M10) → in `2.5.0` (migration `0008`)
 
-- [ ] **Logins lower-cased** at validation (owner, 2026-10-12: fold, do not
+- [x] **Logins lower-cased** at validation (owner, 2026-10-12: fold, do not
    refuse) and `applications.login COLLATE NOCASE` (migration `0008` rebuilds
    the unique index). Before the migration a check
    that no two existing logins differ only in case (there is one