fix(review): address PR review findings for CORE-003

Critical fixes:
- Add math.isfinite() check to reject NaN/Infinity in _safe_quote_price
- Raise TypeError instead of silent 0.0 fallback in price_feed.py
- Use dict instead of Mapping for external data validation

Type improvements:
- Add PortfolioSnapshot TypedDict for type safety
- Add DisplayMode and EntryBasisMode Literal types
- Add explicit dict[str, Any] annotation in to_dict()
- Remove cast() in favor of type comment validation

app/core/calculations.py

@@ -2,7 +2,6 @@ from __future__ import annotations
 
 from collections.abc import Iterable, Mapping
 from datetime import date, datetime
-from typing import cast
 
 from app.core.pricing.black_scholes import (
     DEFAULT_RISK_FREE_RATE,
@@ -153,7 +152,7 @@ def option_row_greeks(
     implied_volatility = float(iv_raw) if isinstance(iv_raw, (int, float)) else 0.0
     volatility = implied_volatility if implied_volatility > 0 else DEFAULT_VOLATILITY
 
-    option_type_typed: OptionType = cast(OptionType, option_type)
+    # option_type is validated to be in {"call", "put"} above, so it's safe to pass
     try:
         pricing = black_scholes_price_and_greeks(
             BlackScholesInputs(
@@ -162,7 +161,7 @@ def option_row_greeks(
                 time_to_expiry=days_to_expiry / 365.0,
                 risk_free_rate=risk_free_rate,
                 volatility=volatility,
-                option_type=option_type_typed,
+                option_type=option_type,  # type: ignore[arg-type]
                 valuation_date=valuation,
             )
         )