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/validation/prediction_snapshot.py

@@ -323,7 +323,29 @@ async def create_prediction_snapshot(
                 "Used positions fallback price for %s: %s", ticker, ticker_price
             )
         else:
-            logger.warning("No market price available for %s at snapshot time", ticker)
+            # Extended fallback: query market_snapshots within 24h time window
+            extended_row = await pool.fetchrow(
+                """SELECT (data->>'c')::float AS close
+                   FROM market_snapshots
+                   WHERE ticker = $1
+                     AND snapshot_type = 'bar'
+                     AND data->>'c' IS NOT NULL
+                     AND captured_at >= NOW() - INTERVAL '24 hours'
+                   ORDER BY captured_at DESC
+                   LIMIT 1""",
+                ticker,
+            )
+            if extended_row:
+                ticker_price = float(extended_row["close"])
+                logger.info(
+                    "Used extended 24h market_snapshots fallback price for %s: %s",
+                    ticker,
+                    ticker_price,
+                )
+            else:
+                logger.warning(
+                    "No market price available for %s at snapshot time", ticker
+                )
 
     spy_price = await fetch_latest_close_price(pool, "SPY")
     if spy_price is None: