Add branch maintenance run command

[joshmgross]

branch maintenance run allows triggering maintenance for a Postgres branch, optionally upgrading to the latest available Postgres minor image for that branch.

https://planetscale.com/docs/postgres/operations-philosophy documents how this works.

[cursor]

Risk: medium. Approved: Cursor Bugbot completed with no findings requiring human review, and Cursor Security Agent did not run on this PR. No reviewers assigned.

  

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