Go-live: docs, changelog, support matrix

[bolshakov]

Background

This is the go-live gate. Until now the ActiveRecord backend is experimental and undocumented on purpose, so the work can land in pieces without committing users to anything. This ticket is the user-facing surface — how to install and configure the backend, what is supported, and the caveats — and it starts only once the backend works and we decide to ship it. Stoplight's support policy documents which dependency versions are tested; the "What We Support" section is the source of truth for that.

Goal

Ship the user-facing surface for the ActiveRecord backend once the decision to release is made.

What to do

• README: an ActiveRecord section beside the Redis one covering setup, the bin/rails <engine>:install:migrations command, the Stoplight.configure snippet, connection guidance (share via ApplicationRecord vs isolate via a dedicated class, and the tradeoffs), experimental status, and caveats (opportunistic pruning, SQLite as dev/test only, run the migration before configuring the backend).
• Add the supported-databases line under "What We Support" naming exactly the databases and versions CI verifies.
• Add a changelog entry.
• Add the ActiveRecord backend to the storage and architecture docs, and to their scope.
• Close Feature request: PostgreSQL (ActiveRecord/SQL) data store adapter #702 with a summary linking this epic.

Acceptance criteria

• The README documents install-to-running-light for the ActiveRecord backend.
• The "What We Support" data-stores list names exactly the databases and versions CI verifies — no version without a CI cell exercising it.
• The storage and architecture docs include the ActiveRecord backend.
• A changelog entry is added.
• Feature request: PostgreSQL (ActiveRecord/SQL) data store adapter #702 is closed with a summary linking this epic.

References

• README.md
• Docs to update: docs/architecture.md, docs/data_storages.md
• CHANGELOG.md