Add OpenAPI changelog tracking API evolution across releases

[nanaf6203-bit]

Area: API Documentation
Priority: P3 — Low

Description

API consumers need to understand what changed between releases. Without a changelog, breaking changes are discovered only after a production failure. Track OpenAPI deltas per release.

Acceptance Criteria

• Generate docs/API_CHANGELOG.md from OpenAPI diffs between consecutive releases
• Each release entry has Added / Changed / Deprecated / Removed / Fixed sections
• Linked from Swagger UI banner when version != latest
• Hooked into release-please workflow (auto-updated on each release)
• First entry seeded manually for v0.1.0 documenting current state
• CI job api-diff verifies non-backward-compatible changes are intentionally tagged

Technical Notes

• Use openapi-diff to compute deltas
• Each breaking change needs explicit confirmation in PR template

[nanaabdul1172]

Hi maintainer I would love to work on this issue
Kindly assign

[grantfox-oss]

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #79)
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