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)

infra/helm/stonks-oracle/values-beta.yaml

@@ -17,6 +17,8 @@ services:
     replicas: 1
   dashboard:
     replicas: 1
+  signalEngine:
+    replicas: 0
 
 ## Beta-specific config overrides
 ## Beta shares the paper DB — DEPLOY_STAGE=beta isolates Redis keys

infra/helm/stonks-oracle/values-paper.yaml

@@ -52,3 +52,5 @@ ingress:
 services:
   extractor:
     replicas: 1
+  signalEngine:
+    replicas: 0

infra/helm/stonks-oracle/values.yaml

@@ -128,7 +128,7 @@ services:
       limits: { cpu: 200m, memory: 128Mi }
 
   signalEngine:
-    replicas: 1
+    replicas: 0
     pipeline: true
     image: signal-engine
     command: "python -m services.signal_engine.main"