fix(coach): feedback references coach_messages, drops plaintext column

Reviewer note: W5 H7 wrote Coach assistant prose plaintext into
recommendation_feedback.recommendation_text. Same prose lives
encrypted in coach_messages — backups captured the leak.

Migration 0042_coach_feedback_message_ref adds nullable coachMessageId
(FK + cascade) and drops recommendation_text. Aggregator joins +
decrypts on read; admin view reads through the same path.

Fixes: Sr-H3 (Wave-6 v1.4.23 senior-dev review)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/ai/feedback-attribution.ts

@@ -87,7 +87,12 @@ export async function resolveFeedbackAttribution(
  * thumbs row. The polymorphic `RecommendationFeedback` table reuses
  * its existing slots for Coach rows:
  *   - `recommendationId` carries the Coach message id
- *   - `recommendationText` carries the assistant prose snapshot
+ *   - `coachMessageId` (added in 0042) FKs the source message so the
+ *     prose stays encrypted-at-rest in `coach_messages`. The senior-
+ *     dev review (Sr-H3) flagged the original H7 design that
+ *     snapshotted decrypted prose into `recommendationText` as a
+ *     silent break of the Coach surface's encryption invariant.
+ *   - `recommendationText` is always NULL for coach rows
  *   - `recommendationSeverity` is always "coach" so dashboards can
  *     filter / group on the literal
  *   - `metricSourceType` encodes the user's active prefs at the