fix(backtest): run backtest asynchronously to prevent WebSocket timeout

- Use run.io_bound() from NiceGUI to run Databento API calls in background thread
- Add loading state to Run Backtest button
- Show notification when backtest starts and completes
- Remove loading state on completion/error

This prevents 'Connection lost' errors when the backtest takes longer than the WebSocket timeout.

app/pages/backtests.py

@@ -5,7 +5,7 @@ from datetime import date, datetime, timedelta
 from typing import Any
 
 from fastapi.responses import RedirectResponse
-from nicegui import ui
+from nicegui import run, ui
 
 from app.domain.backtesting_math import asset_quantity_from_workspace_config
 from app.models.backtest import ProviderRef
@@ -811,8 +811,15 @@ def _render_backtests_page(workspace_id: str | None = None) -> None:
             # Keep existing entry spot, don't re-derive
             mark_results_stale()
 
-        def run_backtest() -> None:
+        async def run_backtest() -> None:
+            """Run the backtest asynchronously to avoid blocking the WebSocket."""
             validation_label.set_text("")
+            
+            # Show loading state
+            run_button.props('loading')
+            validation_label.set_text("Running backtest...")
+            ui.notify("Running backtest...", type="info")
+            
             try:
                 # Validate date range for symbol
                 start_date = parse_iso_date(start_input.value, "Start date")
@@ -830,12 +837,14 @@ def _render_backtests_page(workspace_id: str | None = None) -> None:
                             f"Selected start date {start_date.strftime('%Y-%m-%d')} is before available data."
                         )
                         render_result_state("Invalid start date", validation_label.text, tone="warning")
+                        run_button.props(remove='loading')
                         return
                 
                 date_range_error = validate_date_range_for_symbol(start_date, end_date, symbol)
                 if date_range_error:
                     validation_label.set_text(date_range_error)
                     render_result_state("Scenario validation failed", date_range_error, tone="warning")
+                    run_button.props(remove='loading')
                     return
 
                 # Validate numeric inputs
@@ -846,12 +855,15 @@ def _render_backtests_page(workspace_id: str | None = None) -> None:
                 if numeric_error:
                     validation_label.set_text(numeric_error)
                     render_result_state("Input validation failed", numeric_error, tone="warning")
+                    run_button.props(remove='loading')
                     return
 
                 # Save settings before running
                 save_backtest_settings()
 
-                result = service.run_read_only_scenario(
+                # Run backtest in background thread to avoid blocking WebSocket
+                result = await run.io_bound(
+                    service.run_read_only_scenario,
                     symbol=symbol,
                     start_date=start_date,
                     end_date=end_date,
@@ -864,15 +876,23 @@ def _render_backtests_page(workspace_id: str | None = None) -> None:
                 # Update cost in saved settings after successful run
                 if str(data_source_select.value) == "databento":
                     update_cost_estimate()
+                    
+                render_result(result)
+                run_button.props(remove='loading')
+                validation_label.set_text("")
+                ui.notify("Backtest completed!", type="positive")
+            
             except (ValueError, KeyError) as exc:
+                run_button.props(remove='loading')
                 entry_spot, entry_error = derive_entry_spot()
                 render_seeded_summary(entry_spot=entry_spot, entry_spot_error=entry_error)
                 if entry_spot is None:
                     entry_spot_hint.set_text("Entry spot unavailable until the scenario dates are valid.")
                 validation_label.set_text(str(exc))
                 render_result_state("Scenario validation failed", str(exc), tone="warning")
-                return
+            
             except Exception as exc:
+                run_button.props(remove='loading')
                 entry_spot, entry_error = derive_entry_spot()
                 render_seeded_summary(entry_spot=entry_spot, entry_spot_error=entry_error)
                 if entry_spot is None:
@@ -912,8 +932,6 @@ def _render_backtests_page(workspace_id: str | None = None) -> None:
                     )
                     validation_label.set_text(message)
                     render_result_state("Backtest failed", message, tone="error")
-                return
-            render_result(result)
 
         # Wire up event handlers
         # Only call expensive derive_entry_spot on date changes