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/models/ltv_history.py

@@ -125,7 +125,11 @@ def _require_non_empty_string(data: dict[str, Any], field_name: str) -> str:
 def _decimal_text(value: Decimal) -> str:
     if value == value.to_integral():
         return str(value.quantize(Decimal("1")))
-    return format(value.normalize(), "f") if value.normalize().as_tuple().exponent < 0 else str(value)
+    normalized = value.normalize()
+    exponent = normalized.as_tuple().exponent
+    if isinstance(exponent, int) and exponent < 0:
+        return format(normalized, "f")
+    return str(normalized)
 
 
 def _parse_decimal_payload(

app/models/portfolio.py

@@ -548,29 +548,6 @@ class PortfolioRepository:
             and payload.get("email_alerts") is False
         )
 
-    @classmethod
-    def _serialize_value(cls, key: str, value: Any) -> Any:
-        if key in cls._MONEY_FIELDS:
-            return {"value": cls._decimal_to_string(value), "currency": cls.PERSISTENCE_CURRENCY}
-        if key in cls._WEIGHT_FIELDS:
-            return {"value": cls._decimal_to_string(value), "unit": cls.PERSISTENCE_WEIGHT_UNIT}
-        if key in cls._PRICE_PER_WEIGHT_FIELDS:
-            return {
-                "value": cls._decimal_to_string(value),
-                "currency": cls.PERSISTENCE_CURRENCY,
-                "per_weight_unit": cls.PERSISTENCE_WEIGHT_UNIT,
-            }
-        if key in cls._RATIO_FIELDS:
-            return {"value": cls._decimal_to_string(value), "unit": "ratio"}
-        if key in cls._PERCENT_FIELDS:
-            return {"value": cls._decimal_to_string(value), "unit": "percent"}
-        if key in cls._INTEGER_FIELDS:
-            return cls._serialize_integer(value, unit="seconds")
-        if key == "positions" and isinstance(value, list):
-            # Already serialized as dicts from _to_persistence_payload
-            return value
-        return value
-
     @classmethod
     def _deserialize_value(cls, key: str, value: Any) -> Any:
         if key in cls._MONEY_FIELDS: