tag v3.7.0: patchset review

Patchsets are how contributions reach sourcehut, and Hutch could not see them
at all. This adds review and triage: read a series with its cover letter and
per-patch diffs, see its checks and version chain, and set its status.

Two schema facts shaped the design. MailingList exposes no patchsets field, so
a list's patchsets are reachable only through thread roots — the Patches tab is
derived from the existing threads query and costs no extra request. Patch
carries no diff, so patch bodies are split with the same splitter the inbox
uses and rendered through the existing DiffView.

Patch submission stays out of scope: it is a git send-email flow, not a
GraphQL mutation.