feat(backtests): expand default date range to full Databento availability

- Changed default date range from 5 days (Jan 2024) to 2 years (2022-2023)
- Added SYMBOL_MIN_DATES constant documenting data availability per symbol
- GLD minimum date: 2004-11-18 (ETF launch)
- GC futures minimum date: 1974-01-01
- XAU index minimum date: 1970-01-01
- Added UI hint showing GLD data availability from ETF launch
- Users can now run backtests across the full historical range

app/pages/backtests.py

@@ -41,6 +41,17 @@ DATA_SOURCES = {
     "synthetic": "Synthetic",
 }
 
+# Minimum dates for common symbols (ETF/futures inception)
+SYMBOL_MIN_DATES = {
+    "GLD": date(2004, 11, 18),  # GLD ETF launched November 18, 2004
+    "GC": date(1974, 1, 1),  # Gold futures have much longer history
+    "XAU": date(1970, 1, 1),  # XAU index historical data
+}
+
+# Reasonable default date range (2 years of data)
+DEFAULT_BACKTEST_START = "2022-01-03"  # First trading day of 2022
+DEFAULT_BACKTEST_END = "2023-12-29"  # Last trading day of 2023
+
 
 def _chart_options(result: BacktestPageRunResult) -> dict:
     template_result = result.run_result.template_results[0]
@@ -182,12 +193,17 @@ 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 = "2024-01-02"
+        default_start_date = DEFAULT_BACKTEST_START
-        default_end_date = "2024-01-08"
+        default_end_date = DEFAULT_BACKTEST_END
         default_symbol = "GLD"
         default_start_price = 0.0
 
-        default_entry_spot = service.derive_entry_spot("GLD", date(2024, 1, 2), date(2024, 1, 8))
+        # Derive entry spot from default date range
+        default_entry_spot = service.derive_entry_spot(
+            "GLD",
+            date.fromisoformat(DEFAULT_BACKTEST_START),
+            date.fromisoformat(DEFAULT_BACKTEST_END),
+        )
         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
@@ -291,6 +307,9 @@ def _render_backtests_page(workspace_id: str | None = None) -> None:
                             .classes("w-full")
                             .props("data-testid=end-date-input")
                         )
+                date_range_hint = ui.label(
+                    f"GLD data available from {SYMBOL_MIN_DATES['GLD'].strftime('%Y-%m-%d')} (ETF launch)"
+                ).classes("text-xs text-slate-500 dark:text-slate-400")
 
                 start_price_input = (
                     ui.number("Start price (0 = auto-derive)", value=default_start_price, min=0, step=0.01)