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/instruments.py

@@ -133,7 +133,7 @@ class InstrumentMetadata:
         return Weight(amount=quantity.amount * self.weight_per_share.amount, unit=self.weight_per_share.unit)
 
     def asset_quantity_from_weight(self, weight: Weight) -> AssetQuantity:
-        normalized_weight = weight.to_unit(self.weight_per_share.unit)
+        normalized_weight = weight.to_unit(self.weight_per_share._unit_typed)
         if self.weight_per_share.amount <= 0:
             raise ValueError("Instrument weight_per_share must be positive")
         return AssetQuantity(amount=normalized_weight.amount / self.weight_per_share.amount, symbol=self.symbol)