feat(CONV-001): add GLD launch date validation, feat(DATA-DB-003): add cache CLI

CONV-001:
- Add GLD_LAUNCH_DATE constant (November 18, 2004)
- Validate reference_date in gld_ounces_per_share()
- Raise ValueError for dates before GLD launch
- Update docstring with valid date range
- Add comprehensive test coverage for edge cases

DATA-DB-003:
- Create scripts/cache_cli.py with three commands:
  - vault-dash cache stats: Show cache statistics
  - vault-dash cache list: List cached entries
  - vault-dash cache clear: Clear all cache files
- Add Makefile targets: cache-stats, cache-list, cache-clear
- Integrate with DatabentoHistoricalPriceSource methods

app/services/backtesting/ui_service.py

@@ -315,11 +315,13 @@ class BacktestPageService:
         source_info = self.DATA_SOURCE_INFO.get(data_source, self.DATA_SOURCE_INFO["synthetic"])
 
         # Use the injected provider's identity if available (for custom providers in tests)
-        if hasattr(self.backtest_service, 'provider'):
-            injected_provider_id = getattr(self.backtest_service.provider, 'provider_id', None)
-            injected_pricing_mode = getattr(self.backtest_service.provider, 'pricing_mode', None)
+        if hasattr(self.backtest_service, "provider"):
+            injected_provider_id = getattr(self.backtest_service.provider, "provider_id", None)
+            injected_pricing_mode = getattr(self.backtest_service.provider, "pricing_mode", None)
             # Only use injected identity if it differs from known providers
-            if injected_provider_id and injected_provider_id not in [info.provider_id for info in self.DATA_SOURCE_INFO.values()]:
+            if injected_provider_id and injected_provider_id not in [
+                info.provider_id for info in self.DATA_SOURCE_INFO.values()
+            ]:
                 provider_id = injected_provider_id
                 pricing_mode = injected_pricing_mode or source_info.pricing_mode
             else: