ci: add a force input to the star history floor

[av1155]

Summary

The truncation guard in the star history workflow cannot recover once it trips.
It refuses to publish when the fresh count is under half the published count,
and it reads that published count back out of the chart's own aria-label. A
run that refuses never publishes, so the number it compares against never
moves, and every later run fails identically. workflow_dispatch took no
inputs, so the only way out was hand-editing the aria-label on the assets
branch.

The guard itself is right: a truncated API response should not overwrite a good
chart with a near-empty one. But a legitimate halving, from a mass unstar or a
repo transfer, is indistinguishable from truncation at that point and needs a
way through.

Closes #726

Changes

• workflow_dispatch gains a force boolean input, default false, passed to
  the step through env: rather than interpolated into the script.
• force skips only the floor comparison. The zero-row refusal stays
  unconditional, because publishing the "No stars yet" placeholder over a good
  chart is the exact accident the guard exists to prevent and no legitimate
  force case wants it.
• The refusal message now says how to get past it.
• A missing published chart fails with an ::error:: annotation. Previously
  sed exited non-zero under set -euo pipefail and the step died with a bare
  exit code that reads like an unrelated failure.

Testing

actionlint clean. I ran the guard logic as a standalone script across all six
paths, including the one that is easy to get wrong: on schedule and watch
there are no inputs, so FORCE expands to an empty string, and it has to stay
harmless under set -u.

FORCE	count	published	result
empty (schedule/watch)	264	262	publish
empty	100	262	refuse, below floor
empty	0	262	refuse, zero rows
false	100	262	refuse, below floor
true	100	262	publish
true	0	262	refuse, zero rows

After merge this wants one plain dispatch to confirm the normal path still
publishes.

Type of Change

• Bug fix (fix:)
• New feature (feat:)
• Refactoring (refactor:)
• Documentation (docs:)
• CI/CD (ci:)
• Chore (chore:)

Checklist

• Issue exists and is linked above with Closes #N
• Linked issue has exactly one type:* and one priority:* label
• Linked issue has at most one phase:* label (or none when not roadmap work)
• Branch name matches issue scope (feat/<slug>, fix/<slug>, etc.)
• Tests added/updated for all changes — N/A, workflow shell; verified as above
• Type check passes (mypy src/) — N/A, no Python changed
• Lint passes (ruff check .) — N/A, no Python changed
• Format passes (ruff format --check .) — N/A, no Python changed
• Documentation updated (if applicable) — N/A
• No secrets or sensitive data committed
• Scope check: This change helps search for missing or cutoff-unmet media in a controlled way — N/A, repo tooling