deepen OHLCV history + make the factor-IC pass honest about overlap/regime

Two changes so the cross-sectional signal results can actually be trusted.

(a) History depth — the binding constraint. Ingestion defaulted to 365 days, so
long-lookback factors (12-month momentum, 52-week high) were only computable on a
handful of weeks at the tail, and every IC reflected a single market regime.
- New `settings.ohlcv_history_days` (default 1825 ≈ 5y); new tickers backfill this
  far instead of 1 year.
- New manual "data_backfill" job (Admin → Jobs) re-fetches the full window for
  every ticker, ignoring incremental resume — run once to deepen existing
  1-year histories. Idempotent (upsert); resumes after rate limits.

(b) Factor-IC honesty. The IC was averaged over weekly rebalances whose 30-day
forward windows overlap, inflating the t-stat ~sqrt(6)x.
- IC now measured on NON-OVERLAPPING windows (weeks thinned to ~HORIZON apart).
- Each signal carries a `reliable` flag (>= 12 independent windows); BacktestPanel
  greys out and de-stars thin signals so a lucky 9-week IC of 0.3 can't masquerade
  as an edge.

332 backend tests pass; frontend build clean. No migration (config + job + an
added JSON field on the cached backtest report).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/services/ingestion_service.py

@@ -12,6 +12,7 @@ from datetime import date, timedelta
 from sqlalchemy import func, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
+from app.config import settings
 from app.exceptions import NotFoundError, ProviderError, RateLimitError
 from app.models.ohlcv import OHLCVRecord
 from app.models.settings import IngestionProgress
@@ -92,20 +93,23 @@ async def fetch_and_ingest(
     if end_date is None:
         end_date = date.today()
 
-    # Resolve start_date: use progress resume or default to 1 year ago.
-    # If we have too little history, force a one-year backfill even if
-    # ingestion progress exists (upsert makes this safe and idempotent).
+    # Resolve start_date: use progress resume or backfill the configured history
+    # window. If we have too little history, force a full backfill even if
+    # ingestion progress exists (upsert makes this safe and idempotent). A caller
+    # that passes an explicit start_date (e.g. the manual deep-backfill job)
+    # bypasses this entirely.
     if start_date is None:
         progress = await _get_progress(db, ticker.id)
         bar_count = await _get_ohlcv_bar_count(db, ticker.id)
         minimum_backfill_bars = 200
+        backfill_start = end_date - timedelta(days=settings.ohlcv_history_days)
 
         if bar_count < minimum_backfill_bars:
-            start_date = end_date - timedelta(days=365)
+            start_date = backfill_start
         elif progress is not None:
             start_date = progress.last_ingested_date + timedelta(days=1)
         else:
-            start_date = end_date - timedelta(days=365)
+            start_date = backfill_start
 
     # If start > end, nothing to fetch
     if start_date > end_date: