feat(backtest): add dataset-specific date validation and better error handling

- Add DATABENTO_DATASET_MIN_DATES for XNAS.BASIC (2024-07-01) and GLBX.MDP3 (2010-01-01)
- Validate start date against dataset minimum before running backtest
- Parse Databento API errors and show user-friendly messages
- Update date range hint to show dataset-specific availability
- Catch BentoClientError and show appropriate warning tone

app/pages/backtests.py

@@ -48,6 +48,12 @@ SYMBOL_MIN_DATES = {
     "XAU": date(1970, 1, 1),  # XAU index historical data
 }
 
+# Minimum dates for Databento datasets (when data became available)
+DATABENTO_DATASET_MIN_DATES = {
+    "XNAS.BASIC": date(2024, 7, 1),  # XNAS.BASIC data available from July 2024
+    "GLBX.MDP3": date(2010, 1, 1),  # GLBX.MDP3 futures data from 2010
+}
+
 def get_default_backtest_dates() -> tuple[date, date]:
     """Get default backtest date range (~2 years ending on most recent Friday or earlier).
     
@@ -446,8 +452,21 @@ def _render_backtests_page(workspace_id: str | None = None) -> None:
             return "GLD"  # Default for XNAS.BASIC
 
         def update_date_range_hint() -> None:
-            """Update the date range hint based on selected symbol."""
+            """Update the date range hint based on selected symbol and data source."""
             symbol = get_symbol_from_dataset()
+            data_source = str(data_source_select.value)
+            
+            # Use dataset-specific minimum for Databento
+            if data_source == "databento":
+                dataset = str(dataset_select.value)
+                min_date = DATABENTO_DATASET_MIN_DATES.get(dataset)
+                if min_date:
+                    date_range_hint.set_text(
+                        f"{dataset} data available from {min_date.strftime('%Y-%m-%d')}"
+                    )
+                    return
+            
+            # Fall back to symbol minimum
             min_date = SYMBOL_MIN_DATES.get(symbol)
             if min_date:
                 date_range_hint.set_text(
@@ -805,6 +824,20 @@ def _render_backtests_page(workspace_id: str | None = None) -> None:
                 start_date = parse_iso_date(start_input.value, "Start date")
                 end_date = parse_iso_date(end_input.value, "End date")
                 symbol = get_symbol_from_dataset()
+                
+                # Validate dataset-specific minimum dates for Databento
+                data_source = str(data_source_select.value)
+                if data_source == "databento":
+                    dataset = str(dataset_select.value)
+                    dataset_min = DATABENTO_DATASET_MIN_DATES.get(dataset)
+                    if dataset_min and start_date < dataset_min:
+                        validation_label.set_text(
+                            f"Start date must be on or after {dataset_min.strftime('%Y-%m-%d')} for {dataset} dataset. "
+                            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")
+                        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)
@@ -845,11 +878,32 @@ def _render_backtests_page(workspace_id: str | None = None) -> None:
                 validation_label.set_text(str(exc))
                 render_result_state("Scenario validation failed", str(exc), tone="warning")
                 return
-            except Exception:
+            except Exception as exc:
                 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.")
+                
+                # Check for Databento API errors
+                error_msg = str(exc)
+                if "data_start_before_available_start" in error_msg:
+                    # Extract the available start date from the error message
+                    import re
+                    match = re.search(r"available start of dataset [^(]+\('([^']+)'\)", error_msg)
+                    if match:
+                        available_start = match.group(1).split()[0]  # Extract date part
+                        validation_label.set_text(
+                            f"Data not available before {available_start}. Please set start date to {available_start} or later."
+                        )
+                    else:
+                        validation_label.set_text(
+                            "Selected start date is before data is available for this dataset. Please choose a later date."
+                        )
+                    render_result_state("Invalid start date", validation_label.text, tone="warning")
+                elif "BentoClientError" in error_msg or "422" in error_msg:
+                    validation_label.set_text(f"Data source error: {error_msg}")
+                    render_result_state("Data unavailable", validation_label.text, tone="warning")
+                else:
                     message = "Backtest failed. Please verify the scenario inputs and try again."
                     logger.exception(
                         "Backtest page run failed for workspace=%s symbol=%s start=%s end=%s template=%s units=%s loan=%s margin_call_ltv=%s",