Automate weekly/fortnightly patch releases (currently ad hoc, workflow_dispatch-only)

## Problem

Releases are currently ad hoc: `release.yml` is `workflow_dispatch`-only, so a
tag only goes out when a maintainer remembers to click it. Recent gaps have
ranged from 5 days to ~5 weeks between tags.

At current merge volume (19-24 merged PRs/week over the last several weeks,
18-31 commits/week), this means real backlog accumulates between releases.
As of writing, 33 PRs are merged and unreleased since v2.4.1 (2026-08-04),
including a few security-relevant fixes (YAML frontmatter injection RCE,
missing bearer-token check on a sensitive endpoint, a CVE dependency pin)
that have been sitting unreleased for 2-3 weeks.

## Proposal

1. Add a `schedule:` trigger to `release.yml` for a weekly **patch** release
(default `patch` bump when the run wasn't manually dispatched), gated by a
"skip if nothing merged since the last tag" check so it never produces an
empty release.
2. Keep `workflow_dispatch` exactly as-is for manual `patch`/`minor`/`major`
releases and the existing TestPyPI -> smoke-test -> maintainer-approval-gate
pipeline unchanged — this only adds a second, automatic trigger for the
patch case.
3. Separately: consider an out-of-band trigger (e.g. a `security` label on a
merged PR) that fires an immediate patch release regardless of the weekly
schedule, so security fixes don't wait on a fixed cadence.

A draft diff for (1) exists and has been sanity-checked but not yet run in CI
or merged — happy to open a PR once there's agreement on cadence/day and on
whether (3) belongs in the same change or a follow-up.

## Open questions

- Weekly patch, or fortnightly given how automated the pipeline already is?
- Day/time for the scheduled run?
- Is (3) (security-triggered release) in scope for this issue or a separate one?

Start by reading .github/workflows/release.yml and reviewing the draft diff mentioned in the issue. Confirm how workflow_dispatch handles patch, minor, and major releases, then decide the schedule cadence, run time, and whether the security-triggered release is separate. Done means scheduled patch runs skip when no changes have merged since the last tag while the existing manual and approval pipeline remains unchanged.

Written by the indexing model from the issue text.