Reject-and-detach option for mission submissions

[dhamariT]

What & why

On /admin/missions/<mission>/submissions the reviewer could only Approve or Reject. When a submission was rejected the project stayed attached to that mission, so the builder was stuck on the mission they'd just been rejected from with no clean way to move on.

This adds a third path: reject and detach the project from the mission, freeing the builder to pick a different mission.

The flow

The verdict form now has a "Also detach this project from the mission" checkbox. It only takes effect on rejections (ignored on approve):

• Plain reject — rejects the submission, project keeps its mission. Unchanged behaviour.
• Reject + detach — rejects and detaches the project from the mission so the builder can pick a different one.

How it works

• Detach reuses the existing Project#detach_mission! (soft-detaches the MissionAttachment).
• The detach is guarded: it only fires while the rejected mission is still the project's current mission, so a mission the builder has since swapped to is never yanked.
• Runs inside the existing reject transaction.

Audit / papertrail

• Project::MissionAttachment has has_paper_trail, so the detach is versioned, with whodunnit set via the admin controller chain (attributed to the reviewer).

No migration — reuses the existing soft-detach column.

Tests

• Plain reject → project keeps mission
• Reject + detach → project freed from the mission
• Detach flag ignored when approving

3 runs, 12 assertions, 0 failures, 0 errors

(Rubocop clean on the changed files.)

Notes

• Undo: undo moves a submission back to pending but does not re-attach the mission. Detach is intentionally a one-way reviewer action; flag if you want undo to reverse it too.
• Fallback mission: detach_mission! re-attaches a fallback if the project previously shipped (non-rejected) to another mission — a shipped project never goes mission-less. For a typical first rejection there's no fallback, so the project just becomes mission-free.

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Note

Medium Risk
Changes mission attachment state on reject (guarded to current mission only) inside the existing transaction; undo does not re-attach per PR notes.

Overview
Mission reviewers can optionally detach the builder's project from the mission when rejecting a submission, so the builder is not stuck on a mission they failed.

The review verdict form adds an "Also detach this project from the mission" checkbox (with hint copy and styling). On reject, if that flag is set, the update action calls new detach_submission_project!, which uses existing Project#detach_mission! only when the submission's mission is still the project's current mission. Approve ignores the flag; plain reject behavior is unchanged. The flash notice reflects when a detach actually occurred.

Controller tests cover plain reject, reject+detach, and detach ignored on approve.

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[qcoral]

@dhamariT this should have a demo of it working!