backtest: raise session clamp 45 -> 300 (env BACKTEST_MAX_SESSIONS)

The stored minute-bar history is 273 sessions (~13 months); the 45-session
clamp blocked months-scale ranges now that the nightly labeler keeps
setup_events current. Requests stay serialized behind _backtest_lock; a
full-history run takes minutes by operator choice.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

api/main.py

@@ -581,7 +581,10 @@ class DashboardState:
         from strategy.exits import ExitConfig
         from strategy.population import PopulationConfig
 
-        MAX_SESSIONS = 45
+        # cap protects the (serialized) request thread; 300 spans the full stored
+        # minute-bar history (~273 sessions) so months-scale ranges work via
+        # since/until — a full-history request runs minutes, operator's choice.
+        MAX_SESSIONS = int(os.environ.get("BACKTEST_MAX_SESSIONS", "300"))
         pop = PopulationConfig.from_env()
         regime_on = os.environ.get("TRADING_REGIME_GATE_ENABLED", "0").strip().lower() in {
             "1", "true", "yes", "on"}