test: serialise test files (fileParallelism:false) to avoid shared-DB deadlock

The numbering and manual-create suites both lock the number_sequences
'shared' row (SELECT ... FOR UPDATE + deleteMany); running test files in
parallel raced them into a MySQL 1213 deadlock. Production never does that
concurrent delete, so this is purely a parallel-test isolation fix.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

vitest.config.ts

@@ -10,5 +10,11 @@ export default defineConfig({
     testTimeout: 15000,
     hookTimeout: 15000,
     exclude: ["dist/**", "node_modules/**", ".claude/**"],
+    // Tests run against a real shared MySQL test DB. Running test files in
+    // parallel lets two files (e.g. numbering + manual-create) hit the same
+    // `number_sequences` row with SELECT ... FOR UPDATE + deleteMany at once,
+    // which deadlocks (MySQL 1213). Serialise files so DB-touching suites
+    // don't contend. Tests within a file already run sequentially.
+    fileParallelism: false,
   },
 });