feat: give admin CSAT its own db key + metric

[bookwormsuf]

Problem

The new admin satisfaction measure (1-5 stars) is stored in the same rating
field as legacy thumbs feedback (0/1), and reported via a temporary .rating.v2
metric. Old and new data share one field, and the metric name doesn't say what
it measures.

Solution

Give the new measure its own csat key so it never mixes with legacy rating
data, and name the metric for what it is (CSAT).

Improvements:

• Add optional csat (1-5) field to the admin feedback schema; relax legacy
  rating to optional (kept for historical thumbs rows).
• New rows store csat only; rating is no longer written.
• Rename Datadog metric .rating.v2 → formsg.users.feedback.csat; keep the
  transitional formsg.users.feedback.rating emit so the current report card
  keeps working.
• Backend-only: the wire payload stays rating and is mapped to csat in the
  controller, so no frontend change and no overlap with the star-UI PR.
• No data migration: the flag is off in prod and no star rows exist yet.

Follow-up to the admin satisfaction revamp stack:

1. Feature flag (feat: gate admin feedback behind feature flag #9683)
2. Backend schema + validation (feat: extend admin feedback schema for 1-5 rating + triggers #9684)
3. Zustand store + move modal to builder (feat: zustand store + move feedback modal to builder page #9688)
4. Publish + workflow triggers (feat: add publish and workflow feedback triggers #9689)
5. Star rating UI (feat: replace thumbs up/down with 5-star rating UI #9692)
6. CSAT key + metric rename (this PR)

Breaking Changes

No. csat and rating are both optional; legacy rows keep their rating. The
wire contract is unchanged (still accepts rating). New rows return csat
instead of rating; the workspace UI only reads _id from the response, so
nothing breaks.

Tests

• pnpm build passes
• POST rating 3: DB record has csat: 3, no rating
• formsg.users.feedback.csat fires on create (tag csat); transitional formsg.users.feedback.rating still fires
• PATCH new rating: DB record has csat updated and ratingChanged: true
• PATCH only comment: ratingChanged stays false
• Admin-feedback unit + route specs pass (updated to assert csat)

Automated: service spec 13/13, feedback route spec 13/13, typecheck no new errors, lint clean.

🤖 Generated with Claude Code

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