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/models/portfolio.py

@@ -8,10 +8,14 @@ from dataclasses import dataclass, field
 from datetime import date
 from decimal import Decimal
 from pathlib import Path
-from typing import Any
+from typing import Any, Literal
 
 from app.models.position import Position, create_position
 
+# Type aliases for display mode and entry basis
+DisplayMode = Literal["GLD", "XAU"]
+EntryBasisMode = Literal["value_price", "weight"]
+
 _DEFAULT_GOLD_VALUE = 215_000.0
 _DEFAULT_ENTRY_PRICE = 2_150.0
 _LEGACY_DEFAULT_ENTRY_PRICE = 215.0
@@ -102,7 +106,7 @@ class PortfolioConfig:
     gold_value: float | None = None
     entry_price: float | None = _DEFAULT_ENTRY_PRICE
     gold_ounces: float | None = None
-    entry_basis_mode: str = "value_price"
+    entry_basis_mode: EntryBasisMode = "value_price"
     loan_amount: float = 145000.0
     margin_threshold: float = 0.75
     monthly_budget: float = 8000.0
@@ -117,7 +121,7 @@ class PortfolioConfig:
     underlying: str = "GLD"
 
     # Display mode: how to show positions (GLD shares vs physical gold)
-    display_mode: str = "XAU"  # "GLD" for share view, "XAU" for physical gold view
+    display_mode: DisplayMode = "XAU"  # "GLD" for share view, "XAU" for physical gold view
 
     # Alert settings
     volatility_spike: float = 0.25
@@ -301,7 +305,7 @@ class PortfolioConfig:
         assert self.gold_ounces is not None
         # Sync legacy fields from positions before serializing
         self._sync_legacy_fields_from_positions()
-        result = {
+        result: dict[str, Any] = {
             "gold_value": self.gold_value,
             "entry_price": self.entry_price,
             "gold_ounces": self.gold_ounces,