fix: backfill recommendation evidence for existing recommendations

Migration 028: For each recommendation with no evidence rows, finds
the closest matching trend_window (by ticker + time_horizon + timestamp)
and re-inserts evidence from top_supporting/opposing_evidence arrays.
Filters out non-UUID pattern IDs and verifies documents exist.

This fixes 'No evidence linked' on recommendations created before the
UUID filtering fix in persist_recommendation.

.kiro/steering/project-context.md

@@ -76,7 +76,7 @@ When a full reset is needed:
 ## Database Migrations
 - Located in `infra/migrations/001_*.sql` through `027_*.sql`
 - Applied automatically by `runmefirst.sh` in sorted order
-- Next migration number: **028**
+- Next migration number: **029**
 - Key migrations:
   - 016: Global news interpolation (global_events, macro_impact_records, exposure_profiles, trend_projections)
   - 017: Competitive intelligence (competitor_relationships, competitive_signal_records)