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/migrations/0008_tenant_boundary.sql

@@ -0,0 +1,11 @@
+-- The tenant boundary (audit 2026-10-12, stage 2).
+
+-- An application login is unique without regard to case. Postfix compares
+-- the SASL login with the sender map's without regard to case, so "Ops" and
+-- "ops" were two applications to the panel and one sender to Postfix: a
+-- tenant could take another's envelope sender (audit H4). New logins are
+-- folded to lower case by the panel; a login stored earlier keeps its case,
+-- since its SASL account and its client use it. The column's own UNIQUE
+-- (case-sensitive) stays: SQLite cannot change a column's collation without
+-- rebuilding the table, and this index is the stricter rule.
+CREATE UNIQUE INDEX idx_applications_login_nocase ON applications (login COLLATE NOCASE);