fix: pipeline health — stuck docs, price fallback, sentiment normalization, signal-engine scale, quality gate

- Scheduler: lower stale threshold 240→30 min, batch limit 100→500, TTL 14400→3600
- Prediction snapshot: add 24h market_snapshots time-window fallback
- Aggregation: add normalize_impact_scores() z-score normalization
- Helm: signal-engine replicas → 0 (idle when dual pipeline disabled)
- Quality gate: max_snapshot_age_hours 24→48
- Add backfill script for NULL price_at_prediction snapshots
- Add PBT bug condition and preservation tests (14 tests)

.kiro/specs/pipeline-health-fixes/.config.kiro

@@ -0,0 +1 @@
+{"specId": "f5d99301-94ef-4dc2-8ba4-ccefeee7ecba", "workflowType": "requirements-first", "specType": "bugfix"}