fix: end client before removeAllListeners in stop() to prevent subscribe() hang

[kibae]

Fixes #172

Problem

When stop() was called, removeAllListeners() stripped the end/error handlers from the pg client before client.end() was called. This caused the subscribe() promise to never resolve or reject — hanging the caller indefinitely.

Fix

Call client.end() first, then remove listeners. This ensures the pg query promise inside subscribe() is properly notified when the connection closes.

// Before (broken)
this._client?.removeAllListeners(); // removes end handler
await this._client?.end();          // nobody listening → hang

// After (fixed)
await this._client?.end();          // closes connection, promise resolves
this._client?.removeAllListeners(); // clean up after