fix(cli): bound shutdown with active SSE

[xingkaixin]

Summary

• abort every active SSE stream when server shutdown begins
• bound graceful HTTP shutdown and force-close lingering connections after one second
• always shut down the scan store even if HTTP closing fails

Root cause

server.close() waited for existing connections before the store was shut down. The SSE endpoint kept its response and heartbeat alive indefinitely, so an open browser prevented the close callback and left workers and SQLite resources running.

Verification

• added an integration test that shuts down with an attached SSE client
• verified SSE subscriptions close before the client disconnects
• pnpm --filter codesesh lint
• pnpm --filter codesesh format:check
• pnpm --filter codesesh test
• pnpm --filter codesesh build

Issue: CS-187