Fix hub SQLite cleanup on Windows

[lekoOwO]

Summary

• add an explicit Store.close() method for Bun SQLite-backed hub stores
• close file-backed stores in migration and restart tests before deleting temporary directories
• force a Windows GC cycle after close so unreachable prepared statements release SQLite WAL/SHM file handles before cleanup

Why

Windows refuses to remove a temporary directory while SQLite still has WAL/SHM handles open. The affected hub tests created file-backed stores and then deleted the temp directory without explicitly closing the database.

Validation

• bun run test -- src/store/migration-v8.test.ts src/store/migration-v9.test.ts src/sync/messageService.test.ts in hub
• bun run typecheck:hub

[github-actions]

Findings
No issues found.

Summary
Review mode: initial
No correctness, security, data loss, performance, or maintainability findings met the reporting threshold in the latest diff. Residual risk: I could not execute the Bun test suite in this runner because bun is not installed.

Testing
Not run: bun is not installed in this runner.

HAPI Bot