Add walk-forward backtest harness + paginated history seeder

- data_provider: new fetch_historical() that paginates /time_series in
  5000-bar chunks (1 credit/chunk) for arbitrary date ranges. Tolerates
  the API's "no data on specified dates" boundary error gracefully when
  prior chunks succeeded — that error is just the marker that we walked
  past the symbol's earliest available bar.
- seed_history.py: one-shot CLI that downloads OHLCV for the configured
  pairs/TFs into history/<PAIR>_<TF>.csv. Skips existing files unless
  --force.
- backtest.py: walk-forward replay through SignalAggregator with strict
  no-look-ahead semantics. Each bar is only visible to the engine after
  it has fully closed (`df.index + interval <= as_of`). Trade simulation
  is first-touch (SL vs TP1) with worst-case (SL first if both touched
  same bar), spread cost deducted from R, max-hold timeout, no-overlap.
  Reports per-grade win-rate / avg-R / profit-factor / max-DD, plus
  optional matplotlib equity + by-grade plots.
- requirements: add matplotlib (lazy-imported, only needed for --plot).
- .gitignore: history/ and backtest_results/ regenerable, kept untracked.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitignore

@@ -15,3 +15,7 @@ venv/
 # OS
 .DS_Store
 Thumbs.db
+
+# Backtest data + outputs (regenerable from seed_history.py / backtest.py)
+history/
+backtest_results/