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

tests/test_instruments.py

@@ -10,6 +10,7 @@ from app.domain.instruments import (
     GC_F_OUNCES_PER_CONTRACT,
     GLD_EXPENSE_DECAY_RATE,
     GLD_INITIAL_OUNCES_PER_SHARE,
+    GLD_LAUNCH_DATE,
     GLD_LAUNCH_YEAR,
     Underlying,
     asset_quantity_from_weight,
@@ -22,13 +23,38 @@ from app.domain.instruments import (
 from app.domain.units import BaseCurrency, Weight, WeightUnit
 
 
-def test_gld_ounces_per_share_decay_formula_matches_research() -> None:
-    """Verify decay formula matches research examples from docs/GLD_BASIS_RESEARCH.md."""
-    # Launch (2004): should be exactly 0.10 oz/share
-    launch_backing = gld_ounces_per_share(date(2004, 1, 1))
+def test_gld_launch_date_constant() -> None:
+    """Verify GLD launch date constant is November 18, 2004."""
+    assert GLD_LAUNCH_DATE == date(2004, 11, 18)
+
+
+def test_gld_ounces_per_share_launch_date_returns_initial_backing() -> None:
+    """Verify launch date (2004-11-18) returns exactly 0.10 oz/share."""
+    launch_backing = gld_ounces_per_share(GLD_LAUNCH_DATE)
     assert launch_backing == GLD_INITIAL_OUNCES_PER_SHARE
     assert launch_backing == Decimal("0.10")
 
+
+def test_gld_ounces_per_share_rejects_pre_launch_date() -> None:
+    """Verify dates before GLD launch raise ValueError."""
+    with pytest.raises(ValueError, match="GLD backing data unavailable before"):
+        gld_ounces_per_share(date(2004, 11, 17))  # Day before launch
+
+    with pytest.raises(ValueError, match="GLD backing data unavailable before"):
+        gld_ounces_per_share(date(2004, 1, 1))  # Early 2004
+
+    with pytest.raises(ValueError, match="GLD backing data unavailable before"):
+        gld_ounces_per_share(date(2003, 12, 31))  # Prior year
+
+
+def test_gld_ounces_per_share_early_2004_within_year_raises() -> None:
+    """Verify dates in 2004 but before November 18 also raise ValueError."""
+    with pytest.raises(ValueError, match="GLD backing data unavailable"):
+        gld_ounces_per_share(date(2004, 6, 1))  # June 2004, before launch
+
+
+def test_gld_ounces_per_share_decay_formula_matches_research() -> None:
+    """Verify decay formula matches research examples from docs/GLD_BASIS_RESEARCH.md."""
     # 2026: should be ~0.0916 oz/share (8.4% decay from 22 years)
     # Formula: 0.10 * e^(-0.004 * 22) = 0.10 * e^(-0.088) ≈ 0.091576
     years_2026 = 2026 - GLD_LAUNCH_YEAR  # 22 years