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/scoring.py

@@ -15,6 +15,8 @@ from dataclasses import dataclass, field
 from datetime import datetime, timezone
 from typing import Any
 
+import asyncpg
+
 from services.shared.schemas import MarketContext
 
 # ---------------------------------------------------------------------------
@@ -577,6 +579,57 @@ def sentiment_to_numeric(sentiment: str) -> float:
     return mapping.get(sentiment.lower(), 0.0)
 
 
+async def normalize_impact_scores(
+    pool: asyncpg.Pool,
+    ticker: str,
+    raw_scores: list[float],
+) -> list[float]:
+    """Normalize impact scores using 7-day rolling z-score per ticker.
+
+    Computes the 7-day mean and stddev of impact_score values from
+    document_impact_records for the given ticker, then normalizes
+    each raw score: normalized = (raw - mean_7d) / max(stddev_7d, 0.1)
+
+    The 0.1 floor prevents division by near-zero stddev for low-activity tickers.
+
+    Fallback: if fewer than 5 records in the 7-day window, returns raw scores
+    unchanged (insufficient data for meaningful normalization).
+
+    Args:
+        pool: asyncpg connection pool.
+        ticker: The ticker symbol to query distribution for.
+        raw_scores: List of raw impact_score values to normalize.
+
+    Returns:
+        List of normalized impact scores (same length as input).
+    """
+    if not raw_scores:
+        return []
+
+    row = await pool.fetchrow(
+        """
+        SELECT AVG(impact_score) as mean,
+               STDDEV(impact_score) as stddev,
+               COUNT(*) as cnt
+        FROM document_impact_records
+        WHERE ticker = $1 AND created_at >= NOW() - INTERVAL '7 days'
+        """,
+        ticker,
+    )
+
+    # Fallback: insufficient data for meaningful normalization
+    if row is None or row["cnt"] < 5:
+        return list(raw_scores)
+
+    mean_7d: float = float(row["mean"])
+    stddev_7d: float = float(row["stddev"]) if row["stddev"] is not None and row["stddev"] > 0 else 0.0
+
+    # Apply 0.1 floor to prevent division by near-zero stddev
+    effective_stddev = max(stddev_7d, 0.1)
+
+    return [(raw - mean_7d) / effective_stddev for raw in raw_scores]
+
+
 def weighted_sentiment_average(signals: list[WeightedSignal]) -> float:
     """Compute a weight-adjusted average sentiment across signals.