feat: consolidate ledger trade writes into portfolio_ledger

- add ledger consolidation migration and remove ledger_trades usage
- return portfolio state after v1 ledger trade writes
- deprecate legacy buy/sell endpoints with deprecation headers
- add BAT-016, DCY-006, and REQ-036~038 documentation
- update repository models and tests for unified ledger entries

app/repositories/portfolio_repository.py

@@ -5,7 +5,7 @@ from __future__ import annotations
 from datetime import datetime
 from typing import Protocol
 
-from app.domain.models import Asset, AuthToken, BrokerCredential, Daemon, LedgerEntry, LedgerTrade, Portfolio, PortfolioComment, PortfolioTag, StrategySet, Tag, TokenPrincipal, User
+from app.domain.models import Asset, AuthToken, BrokerCredential, Daemon, LedgerEntry, Portfolio, PortfolioComment, PortfolioTag, StrategySet, Tag, TokenPrincipal, User
 
 
 class PortfolioRepository(Protocol):
@@ -262,7 +262,7 @@ class PortfolioRepository(Protocol):
         executed_at: datetime,
         fee_amount: float = 0.0,
         tax_amount: float = 0.0,
-    ) -> tuple[LedgerTrade, bool]: ...
+    ) -> tuple[LedgerEntry, bool]: ...
 
     def reset(self, portfolio_id: str) -> Portfolio: ...