db: integrate golang-migrate for schema migrations

- internal/db/migrations/: five embedded SQL files containing
  the migration SQL previously held as Go string literals.
  Files follow the NNN_description.up.sql naming convention
  required by golang-migrate's iofs source.
- internal/db/migrate.go: rewritten to use
  github.com/golang-migrate/migrate/v4 with the
  database/sqlite driver (modernc.org/sqlite, pure Go) and
  source/iofs for compile-time embedded SQL.
  - newMigrate() opens a dedicated *sql.DB so m.Close() does
    not affect the caller's shared connection.
  - Migrate() includes a compatibility shim: reads the legacy
    schema_version table and calls m.Force(v) before m.Up()
    so existing databases are not re-migrated.
  - LatestSchemaVersion promoted from var to const.
- internal/db/db.go: added path field to DB struct; Open()
  translates ':memory:' to a named shared-cache URI
  (file:mcias_N?mode=memory&cache=shared) so the migration
  runner can open a second connection to the same in-memory
  database without sharing the handle that golang-migrate
  will close on teardown.
- go.mod: added golang-migrate/migrate/v4 v4.19.1 (direct).
All callers unchanged. All tests pass; golangci-lint clean.

web/templates/login.html

@@ -10,6 +10,7 @@
 <div class="login-wrapper">
   <div class="login-box">
     <div class="brand-heading">MCIAS</div>
+    <div class="brand-subtitle">Metacircular Identity &amp; Access System</div>
     <div class="card">
       {{if .Error}}<div class="alert alert-error" role="alert">{{.Error}}</div>{{end}}
       <form id="login-form" method="POST" action="/login"