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)

services/aggregation/worker.py

@@ -51,6 +51,7 @@ from services.aggregation.scoring import (
     ScoringConfig,
     WeightedSignal,
     compute_signal_weight,
+    normalize_impact_scores,
     sentiment_to_numeric,
     weighted_sentiment_average,
 )
@@ -2312,6 +2313,13 @@ async def aggregate_company_window(
     # 2. Fetch market context
     market_ctx = await fetch_market_context(pool, ticker, window, reference_time)
 
+    # 2a. Normalize impact scores using 7-day z-score per ticker
+    if impacts:
+        raw_scores = [imp.impact_score for imp in impacts]
+        normalized_scores = await normalize_impact_scores(pool, ticker, raw_scores)
+        for imp, norm_score in zip(impacts, normalized_scores):
+            imp.impact_score = norm_score
+
     # 3. Build weighted signals — pass source accuracy and market data
     #    when in probabilistic mode (Req 4.1–4.3, 6.1–6.5)
     signals = build_weighted_signals(