chore: bump simplexmq from 6.2.0 to 6.4.5

[helix-nine]

Summary

• Bumps the simplexmq submodule from v6.2.0 to v6.4.5
• Updates manifest.yaml version from 6.2.0.7 to 6.4.5.1
• Updates release notes with key upstream changes

Upstream Changes (6.2.0 → 6.4.5)

This is a significant minor version update spanning multiple releases with major new features and improvements:

New Features

• PostgreSQL support for queue records — designed for high-traffic servers that need database-backed storage
• Prometheus metrics for both SMP and XFTP servers — enables monitoring and observability
• Short connection links support — simplified link sharing
• Store log enabled by default — improved data durability out of the box
• Content moderation blocking records

Improvements

• Multiple memory leak fixes across several releases
• Stability and reliability improvements
• Various bug fixes

Proposed Features to Expose

The following upstream features could be exposed to StartOS users in future updates:

1. PostgreSQL backend — Could be offered as an optional configuration for users running high-traffic servers. Would require adding a PostgreSQL dependency or bundling it in the container.
2. Prometheus metrics endpoint — Could be exposed as a new interface for users who want to monitor their server with Grafana or similar tools.
3. Short links configuration — May require new config options to enable/customize.

These features are not implemented in this PR — this PR only bumps the version. The features above are proposed for discussion and follow-up work.

Full Changelog

https://github.com/simplex-chat/simplexmq/blob/master/CHANGELOG.md