fix(types): resolve all mypy type errors (CORE-003)

- Fix return type annotation for get_default_premium_for_product
- Add type narrowing for Weight|Money union using _as_money helper
- Add isinstance checks before float() calls for object types
- Add type guard for Decimal.exponent comparison
- Use _unit_typed and _currency_typed properties for type narrowing
- Cast option_type to OptionType Literal after validation
- Fix provider type hierarchy in backtesting services
- Add types-requests to dev dependencies
- Remove '|| true' from CI type-check job

All 36 mypy errors resolved across 15 files.

app/domain/portfolio_math.py

@@ -30,6 +30,13 @@ def _money_to_float(value: Money) -> float:
     return float(value.amount)
 
 
+def _as_money(value: Weight | Money) -> Money:
+    """Narrow Weight | Money to Money after multiplication."""
+    if isinstance(value, Money):
+        return value
+    raise TypeError(f"Expected Money, got {type(value).__name__}")
+
+
 def _decimal_to_float(value: Decimal) -> float:
     return float(value)
 
@@ -48,7 +55,12 @@ def _gold_weight(gold_ounces: float) -> Weight:
 
 def _safe_quote_price(value: object) -> float:
     try:
-        parsed = float(value)
+        if isinstance(value, (int, float)):
+            parsed = float(value)
+        elif isinstance(value, str):
+            parsed = float(value.strip())
+        else:
+            return 0.0
     except (TypeError, ValueError):
         return 0.0
     if parsed <= 0:
@@ -121,7 +133,7 @@ def _strategy_option_payoff_per_unit(
     return sum(
         weight * max(strike_price - scenario_spot, _DECIMAL_ZERO)
         for weight, strike_price in _strategy_downside_put_legs(strategy, current_spot)
-    )
+    ) or Decimal("0")
 
 
 def _strategy_upside_cap_effect_per_unit(
@@ -233,7 +245,7 @@ def portfolio_snapshot_from_config(
     config: PortfolioConfig | None = None,
     *,
     runtime_spot_price: float | None = None,
-) -> dict[str, float]:
+) -> dict[str, float | str]:
     """Build portfolio snapshot with display-mode-aware calculations.
 
     In GLD mode:
@@ -294,7 +306,7 @@ def portfolio_snapshot_from_config(
         margin_call_ltv = decimal_from_float(float(config.margin_threshold))
         hedge_budget = Money(amount=decimal_from_float(float(config.monthly_budget)), currency=BaseCurrency.USD)
 
-    gold_value = gold_weight * spot
+    gold_value = _as_money(gold_weight * spot)
     net_equity = gold_value - loan_amount
     ltv_ratio = _decimal_ratio(loan_amount.amount, gold_value.amount)
     margin_call_price = loan_amount.amount / (margin_call_ltv * gold_weight.amount)
@@ -334,7 +346,7 @@ def build_alert_context(
         gold_weight = _gold_weight(float(config.gold_ounces or 0.0))
 
     live_spot = _spot_price(spot_price)
-    gold_value = gold_weight * live_spot
+    gold_value = _as_money(gold_weight * live_spot)
     loan_amount = Money(amount=decimal_from_float(float(config.loan_amount)), currency=BaseCurrency.USD)
     margin_call_ltv = decimal_from_float(float(config.margin_threshold))
     margin_call_price = (
@@ -377,12 +389,12 @@ def strategy_metrics_from_snapshot(
     ]
 
     scenario_price = spot * _pct_factor(scenario_pct)
-    scenario_gold_value = gold_weight * PricePerWeight(
+    scenario_gold_value = _as_money(gold_weight * PricePerWeight(
         amount=scenario_price,
         currency=BaseCurrency.USD,
         per_unit=WeightUnit.OUNCE_TROY,
-    )
-    current_gold_value = gold_weight * current_spot
+    ))
+    current_gold_value = _as_money(gold_weight * current_spot)
     unhedged_equity = scenario_gold_value - loan_amount
     scenario_payoff_per_unit = _strategy_option_payoff_per_unit(strategy, spot, scenario_price)
     capped_upside_per_unit = _strategy_upside_cap_effect_per_unit(strategy, spot, scenario_price)