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

Makefile

@@ -1,4 +1,4 @@
-.PHONY: install dev lint test build deploy
+.PHONY: install dev lint test build deploy cache-stats cache-clear cache-list
 
 install:
 	python3 -m venv .venv
@@ -19,3 +19,13 @@ build: lint
 
 deploy:
 	./scripts/deploy.sh
+
+# Cache management commands
+cache-stats:
+	. .venv/bin/activate && python scripts/cache_cli.py stats
+
+cache-list:
+	. .venv/bin/activate && python scripts/cache_cli.py list
+
+cache-clear:
+	. .venv/bin/activate && python scripts/cache_cli.py clear --yes