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/services/price_feed.py

@@ -7,7 +7,6 @@ import logging
 import math
 from dataclasses import dataclass
 from datetime import datetime
-from typing import Mapping
 
 import yfinance as yf
 
@@ -52,15 +51,15 @@ class PriceFeed:
         self._cache = get_cache()
 
     @staticmethod
-    def _required_payload_value(payload: Mapping[str, object], key: str, *, context: str) -> object:
+    def _required_payload_value(payload: dict[str, object], key: str, *, context: str) -> object:
         if key not in payload:
             raise TypeError(f"{context} is missing required field: {key}")
         return payload[key]
 
     @classmethod
     def _normalize_cached_price_payload(cls, payload: object, *, expected_symbol: str) -> PriceData:
-        if not isinstance(payload, Mapping):
-            raise TypeError("cached price payload must be an object")
+        if not isinstance(payload, dict):
+            raise TypeError("cached price payload must be a plain dict")
         payload_symbol = str(payload.get("symbol", expected_symbol)).strip().upper()
         normalized_symbol = expected_symbol.strip().upper()
         if payload_symbol != normalized_symbol:
@@ -69,7 +68,9 @@ class PriceFeed:
         if not isinstance(timestamp, str) or not timestamp.strip():
             raise TypeError("cached timestamp must be a non-empty ISO string")
         price_val = cls._required_payload_value(payload, "price", context="cached price payload")
-        price = float(price_val) if isinstance(price_val, (int, float)) else 0.0
+        if not isinstance(price_val, (int, float)):
+            raise TypeError(f"cached price must be numeric, got {type(price_val).__name__}")
+        price = float(price_val)
         return PriceData(
             symbol=payload_symbol,
             price=price,
@@ -80,8 +81,8 @@ class PriceFeed:
 
     @classmethod
     def _normalize_provider_price_payload(cls, payload: object, *, expected_symbol: str) -> PriceData:
-        if not isinstance(payload, Mapping):
-            raise TypeError("provider price payload must be an object")
+        if not isinstance(payload, dict):
+            raise TypeError("provider price payload must be a plain dict")
         payload_symbol = str(payload.get("symbol", expected_symbol)).strip().upper()
         normalized_symbol = expected_symbol.strip().upper()
         if payload_symbol != normalized_symbol:
@@ -90,7 +91,9 @@ class PriceFeed:
         if not isinstance(timestamp, datetime):
             raise TypeError("provider timestamp must be a datetime")
         price_val = cls._required_payload_value(payload, "price", context="provider price payload")
-        price = float(price_val) if isinstance(price_val, (int, float)) else 0.0
+        if not isinstance(price_val, (int, float)):
+            raise TypeError(f"provider price must be numeric, got {type(price_val).__name__}")
+        price = float(price_val)
         return PriceData(
             symbol=payload_symbol,
             price=price,