feat!: add changeset capabilites to API

[angstwad]

Okay, so this is a massive PR, so apologies.

To make it easier to review, I guess, you can trust the tests a little bit. Or not. I think they're decent. But I made you a "verify the behavior of the API" doc that is in this PR at docs/verification-walkthrough.md. There is also a new user-guide.md there, too, that should help.

Also, I'd suggest you walk through some of the flows in the UI as well... Check out the API endpoints in the OpenAPI page when running locally at http://localhost:8000/api/v1/docs.

And in the UI, you can verify the actions do what they're supposed to do.

I added a ton of documentation on this and in the code to help make review a bit easier.

Summary:
This PR introduces a comprehensive ChangeSet system to the Parameter Store API, enabling atomic, versioned updates to Groups and Clusters. It transitions the API from direct "live" modifications to a staged workflow where changes are drafted, reviewed, and committed as cohesive units.

Key Changes:

• API Architecture:
  • Implemented ChangeSet lifecycle management: creation, metadata updates, abandonment, and atomic committing.
  • Refactored Group and Cluster CRUD endpoints to strictly require a changeset_id for all state-changing operations (POST, PUT, DELETE).
  • Standardized ID-based lookups (/id/{id}) to use stable shared_entity_id (UUID) rather than ephemeral database row IDs.
  • Added a ChangeSet Summary endpoint (GET /changeset/{id}/changes) providing a categorized breakdown (Create/Update/Delete) of all pending modifications.
  • Added History endpoints (/.../history) for Groups and Clusters to browse the version trail of an entity.
• Django Admin Integration:
  • Enhanced the Admin UI with updated custom actions for committing, abandoning, and coalescing ChangeSets.
  • Implemented interception logic to ensure deletions from the Admin UI correctly stage changes into the active ChangeSet.
• Documentation:
  • Created a new docs/user-guide.md providing a clear, approachable walkthrough of the ChangeSet system for both UI and API users.
  • Thoroughly documented the codebase with Google-style docstrings and descriptive Pydantic field definitions for improved OpenAPI/Swagger visibility.
• Testing:
  • Supplemented with an extensive test suite (84+ tests) covering the full lifecycle of ChangeSets, entity versioning, and administrative workflows.