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/domain/instruments.py

@@ -36,6 +36,7 @@ class Underlying(str, Enum):
 GLD_INITIAL_OUNCES_PER_SHARE = Decimal("0.10")
 GLD_EXPENSE_DECAY_RATE = Decimal("0.004")  # 0.4% annual decay
 GLD_LAUNCH_YEAR = 2004
+GLD_LAUNCH_DATE = date(2004, 11, 18)  # GLD IPO date on NYSE
 
 # GC=F contract specifications
 GC_F_OUNCES_PER_CONTRACT = Decimal("100")  # 100 troy oz per contract
@@ -44,29 +45,41 @@ GC_F_QUOTE_CURRENCY = BaseCurrency.USD
 
 def gld_ounces_per_share(reference_date: date | None = None) -> Decimal:
     """
-    Calculate GLD's current gold backing per share based on expense ratio decay.
+    Calculate GLD's gold backing per share for a specific date.
 
     GLD's expense ratio (0.40% annually) causes the gold backing per share to
-    decay exponentially from the initial 0.10 oz/share at launch (2004).
+    decay exponentially from the initial 0.10 oz/share at launch (November 18, 2004).
 
     Formula: ounces_per_share = 0.10 * e^(-0.004 * years_since_2004)
 
     Args:
-        reference_date: Date to calculate backing for. Defaults to today.
+        reference_date: Date to calculate backing for. Must be on or after
+            GLD launch date (2004-11-18). Defaults to today.
 
     Returns:
         Decimal representing troy ounces of gold backing per GLD share.
 
+    Raises:
+        ValueError: If reference_date is before GLD launch (2004-11-18).
+
     Examples:
-        >>> # 2026 backing should be ~0.0919 oz/share (8.1% decay)
         >>> from datetime import date
+        >>> # Launch date returns initial 0.10 oz/share
+        >>> gld_ounces_per_share(date(2004, 11, 18))
+        Decimal('0.10')
+        >>> # 2026 backing should be ~0.0916 oz/share (8.4% decay)
         >>> result = gld_ounces_per_share(date(2026, 1, 1))
         >>> float(result)  # doctest: +SKIP
-        0.0919...
+        0.0916...
     """
     if reference_date is None:
         reference_date = date.today()
 
+    if reference_date < GLD_LAUNCH_DATE:
+        raise ValueError(
+            f"GLD backing data unavailable before {GLD_LAUNCH_DATE}. " f"GLD launched on November 18, 2004."
+        )
+
     years_since_launch = Decimal(reference_date.year - GLD_LAUNCH_YEAR)
     decay_factor = Decimal(str(math.exp(-float(GLD_EXPENSE_DECAY_RATE * years_since_launch))))
     return GLD_INITIAL_OUNCES_PER_SHARE * decay_factor