chore: bump version to 2.2.1

[vishaltps]

Summary

Release prep for v2.2.1 — a patch release shipping the multi-database chart fix from #43.

Changes

• lib/solid_queue_monitor/version.rb: 2.2.0 → 2.2.1
• CHANGELOG.md: promote [Unreleased] → [2.2.1] - 2026-07-13
• Gemfile.lock: refresh solid_queue_monitor pin to 2.2.1 (keeps CI's frozen bundle install green)

Included since 2.2.0

• fix: activity chart no longer crashes the dashboard on multi-database setups where Solid Queue runs on a different engine than the primary database (config.show_chart = true fails with postgres #42, fix: detect chart SQL dialect from queue connection #43)

Testing

• bundle install succeeds and updates the lockfile to 2.2.1
• Full suite green as of fix: detect chart SQL dialect from queue connection #43

Checklist

• Version bumped
• CHANGELOG updated with date
• Gemfile.lock updated