feat: publish Scalar HTTP API reference

[PsiACE]

Which issue or RFC does this PR close?

Closes #1372.

Rationale for this change

Expose the canonical HTTP API alongside the existing Python API reference.

What changes are included in this PR?

• publish Scalar at /api/ and /docs;
• keep /openapi.json as the raw schema endpoint;
• update navigation and documentation builds.

Are there any user-facing changes?

Adds HTTP API reference pages. No breaking API changes.

How was this change tested?

• make check
• make test
• make contract-test
• make docs-test

AI usage statement

OpenAI Codex was used.

[AsperforMias]

may we need to care the e2e ci err?

[PsiACE]

@AsperforMias this e2e err is buggy, and fixed..... There are currently a lot of tests and locks that aren't related to behavior; I'll remove them in another PR.