Add per-topic prompt checklist

[Shalom-Karr]

Add per-topic prompt checklist feature

Staff can attach a checklist of items to a single topic from the topic
admin (wrench) menu via a dedicated "Prompt Checklist" entry (separate
from "Moderator Actions"). When any user, including staff, replies to
that topic, a modal pops up first and they must tick every item before
the reply is allowed through.

Acceptance is recorded per-topic-per-user with a version, so a staff
edit bumps the version and re-prompts previously-accepted users.

Storage:

• Topic custom field mod_topic_prompt_checklist (json): items,
  version, button_label, updated_at.
• User custom field mod_topic_checklist_accepted (json): map of
  topic_id => accepted_version.

Server:

• New staff-gated routes GET/PUT/DELETE /topic/:topic_id/prompt-checklist
  in the existing checklist controller, reusing parse_items for the
  index-keyed-hash shape the browser form-encodes.
• owed_checklist_for(user, topic_id:) now considers per-topic
  checklists when topic_id is supplied; priority: targeted > per-topic

  global. /checklist/owed?topic_id=... carries the new kind:topic
  result with the topic id.

• Existing POST /checklist/accept handles kind:topic and records
  the clamped accepted version into the user map.
• topic_view serializer exposes the per-topic checklist for the
  editor modal's initial load.

Frontend:

• New initializer registers the topic-admin-menu entry via
  addTopicAdminMenuButton.
• New modal mod-topic-prompt-checklist-modal.gjs lets staff load /
  edit / save / clear the per-topic checklist; save bumps version and
  shows a toast.
• Composer gate in precheck-prompt.js passes topic_id to
  refreshOwedChecklist when replying.
• The user-facing modal mod-first-post-checklist.gjs already passes
  kind and id through to /checklist/accept.

Tests:

• spec/requests/topic_prompt_checklist_spec.rb covers staff CRUD,
  403 for regular users, the topic_id query on /checklist/owed
  (per-topic gating + clamp + version bump + priority over global,
  yields to targeted), accept-with-kind-topic, and the topic_view
  serializer.
• spec/system/topic_prompt_checklist_spec.rb walks the staff editor
  flow and the user reply flow end-to-end, with screenshots 171-180.

Docs:

• New docs/topic-prompt-checklist.md.
• README, features.md, testing.md updated.

Co-Authored-By: Claude Opus 4.7 noreply@anthropic.com