Add support for listing all branches of a namespace that is a git root

[shangyian]

Summary

This PR adds a new endpoint (GET /namespaces/{namespace}/branches) that lists all branch namespaces under a git root namespace, returning per-branch health metadata like node count, invalid node count, git-only flag, and the last successful deployment timestamp.

The default branch (e.g. something.main) is always sorted first regardless of parent_namespace convention, which handles namespaces predating the parent_namespace field. All counts are computed in a single query via correlated subqueries.

Test Plan

• PR has an associated issue: #
• make check passes
• make test shows 100% unit test coverage

Deployment Plan

[netlify]

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