fix: register the Apple-domain backfill with the sqlite runner

The sqlite migration list is hand-maintained, so the file alone did nothing —
a fresh boot still reported 87 files and stopped at version 86. MySQL and
Postgres read their directories, so only this one needed the entry.

Caught by booting a local instance rather than by the suite: the client tests
pin `CURRENT_SCHEMA_VERSION`, which fails once the entry exists but says
nothing while the file is simply unregistered.

Verified against a seeded database at the old version. `John.Smith@iCloud.com`
went from `johnsmith@icloud.com` to `john.smith@icloud.com`, `a.b+tag@me.com`
from `ab@me.com` to `a.b@me.com` — subaddressing still folded — and a gmail row
was left alone.

src/backend/clients/database/SqliteDatabaseClient.test.ts

@@ -27,7 +27,7 @@ import { DatabaseClientFactory } from './index.js';
 import { SqliteDatabaseClient } from './SqliteDatabaseClient.js';
 
 /** Highest schema version the migration table can reach. */
-const CURRENT_SCHEMA_VERSION = 86;
+const CURRENT_SCHEMA_VERSION = 87;
 
 /**
  * These suites migrate real files on disk. Idle they finish in well under a

src/backend/clients/database/SqliteDatabaseClient.ts

@@ -120,6 +120,7 @@ const AVAILABLE_MIGRATIONS: [number, string[]][] = [
     [83, ['0088_apps-index-url.sql']],
     [84, ['0089_team-require-2fa.sql']],
     [85, ['0090_user-app-permission-index.sql']],
+    [86, ['0091_icloud-dot-significance.sql']],
 ];
 
 export class SqliteDatabaseClient extends AbstractDatabaseClient {