Add a changelog for the maintained fork

[kantselovich]

Add a changelog for the maintained fork

Is your feature request related to a problem? Please describe.
The project does not currently have a changelog. That makes it harder to understand what changed between releases, what changed since the LOCALiQ-maintained fork diverged from upstream, and which updates may affect users directly.

This is especially relevant for:

• Python version support changes
• database adapter behavior or coverage changes
• CI/tooling migrations
• dependency and security-related updates

Describe the solution you'd like
Add a root-level CHANGELOG.md and use it for release notes going forward.

Suggested format:

• follow a simple Keep a Changelog-style structure
• keep an Unreleased section at the top
• use reverse chronological release entries
• group entries under:
  • Added
  • Changed
  • Fixed
  • Removed
  • Security

Suggested scope:

• start documenting releases for the LOCALiQ-maintained fork
• add a short initial entry that marks the fork/maintenance transition
• use the changelog for user-visible and operationally relevant changes

Suggested validation:

• add a lightweight CI check or script that verifies CHANGELOG.md exists
• validate that it includes at least an Unreleased section and release headings
• optionally require changelog updates for user-visible PRs, or explicitly note when no entry is needed

Describe alternatives you've considered

• Rely only on PR history and GitHub releases
• Start a changelog only for future releases without any fork-era context

Both are lower effort, but a repository changelog would make maintenance history much easier to follow.

Additional context
Acceptance criteria:

• add CHANGELOG.md at the repository root
• define and document a consistent entry format
• include an Unreleased section
• include at least one initial fork-era or maintenance-transition entry
• add a lightweight validation check or documented process so the changelog does not immediately become stale