fix(backtest): handle Databento errors gracefully during page load

- Set default dates to 2024-07-01 to 2024-12-31 (valid for XNAS.BASIC)
- Catch all exceptions during entry spot derivation, not just ValueError
- Don't auto-run backtest on page load - let user configure first
- Use recent GLD price (~30) as fallback

app/pages/backtests.py

@@ -254,24 +254,25 @@ def _render_backtests_page(workspace_id: str | None = None) -> None:
         default_data_source = "databento"
         default_dataset = "XNAS.BASIC"
         default_schema = "ohlcv-1d"
-        default_start_date = DEFAULT_BACKTEST_START
-        default_end_date = DEFAULT_BACKTEST_END
+        # Use a start date that's valid for the default dataset (XNAS.BASIC starts 2024-07-01)
+        default_start_date = date(2024, 7, 1).isoformat()
+        default_end_date = date(2024, 12, 31).isoformat()
         default_symbol = "GLD"
         default_start_price = 0.0
 
         # Derive entry spot from default date range
-        # Fall back to a reasonable default if fixture source doesn't support the date range
+        # Fall back to a reasonable default if data source doesn't support the date range
         try:
             default_entry_spot = service.derive_entry_spot(
                 "GLD",
-                date.fromisoformat(DEFAULT_BACKTEST_START),
-                date.fromisoformat(DEFAULT_BACKTEST_END),
-                data_source="databento",  # Use databento for default dates
+                date.fromisoformat(default_start_date),
+                date.fromisoformat(default_end_date),
+                data_source="databento",
             )
-        except ValueError:
-            # Fixture source may not support the default date range
-            # Fall back to a reasonable GLD price
-            default_entry_spot = 185.0
+        except Exception:
+            # Data source may not support the default date range or API error
+            # Fall back to a reasonable GLD price (recent ~$230/oz)
+            default_entry_spot = 230.0
         default_units = (
             asset_quantity_from_workspace_config(config, entry_spot=default_entry_spot, symbol="GLD")
             if config is not None and default_entry_spot > 0
@@ -937,4 +938,4 @@ def _render_backtests_page(workspace_id: str | None = None) -> None:
 
         # Initial render
         render_seeded_summary(entry_spot=float(default_start_price) if default_start_price > 0 else None)
-        run_backtest()
+        # Don't auto-run backtest on page load - let user configure and click Run