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.

.forgejo/workflows/ci.yaml

@@ -70,5 +70,5 @@ jobs:
       - name: Run mypy
         run: |
           echo "Running mypy on core modules..."
-          mypy app/core app/models app/strategies app/services --ignore-missing-imports --show-error-codes --show-traceback || true
-          echo "Type check completed (warnings allowed during development - see CORE-003 roadmap task)"
+          mypy app/core app/models app/strategies app/services --ignore-missing-imports --show-error-codes --show-traceback
+          echo "Type check completed successfully"