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/services/backtesting/ui_service.py

@@ -15,8 +15,16 @@ from app.models.backtest import (
     TemplateRef,
 )
 from app.services.backtesting.databento_source import DatabentoHistoricalPriceSource, DatabentoSourceConfig
-from app.services.backtesting.fixture_source import bind_fixture_source, build_backtest_ui_fixture_source
-from app.services.backtesting.historical_provider import DailyClosePoint, YFinanceHistoricalPriceSource
+from app.services.backtesting.fixture_source import (
+    FixtureBoundSyntheticHistoricalProvider,
+    SharedHistoricalFixtureSource,
+    build_backtest_ui_fixture_source,
+)
+from app.services.backtesting.historical_provider import (
+    DailyClosePoint,
+    SyntheticHistoricalProvider,
+    YFinanceHistoricalPriceSource,
+)
 from app.services.backtesting.input_normalization import normalize_historical_scenario_inputs
 from app.services.backtesting.service import BacktestService
 from app.services.strategy_templates import StrategyTemplateService
@@ -98,7 +106,10 @@ class BacktestPageService:
         )
         self.template_service = template_service or base_service.template_service
         self.databento_config = databento_config
-        fixture_provider = bind_fixture_source(base_service.provider, build_backtest_ui_fixture_source())
+        fixture_provider = FixtureBoundSyntheticHistoricalProvider(
+            base_provider=SyntheticHistoricalProvider(),
+            source=build_backtest_ui_fixture_source(),
+        )
         self.backtest_service = copy(base_service)
         self.backtest_service.provider = fixture_provider
         self.backtest_service.template_service = self.template_service
@@ -135,11 +146,9 @@ class BacktestPageService:
             List of daily close points sorted by date
         """
         if data_source == "databento":
-            provider = self._get_databento_provider()
-            return provider.load_daily_closes(symbol, start_date, end_date)
+            return self._get_databento_provider().load_daily_closes(symbol, start_date, end_date)
         elif data_source == "yfinance":
-            provider = self._get_yfinance_provider()
-            return provider.load_daily_closes(symbol, start_date, end_date)
+            return self._get_yfinance_provider().load_daily_closes(symbol, start_date, end_date)
         else:
             # Use synthetic fixture data
             return self.backtest_service.provider.load_history(symbol, start_date, end_date)