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

tests/test_v1.py

@@ -11,7 +11,7 @@ from fastapi.testclient import TestClient
 
 from app.core.auth import AuthContext, Permission
 from app.core.config import Settings, get_settings
-from app.domain.models import AuthToken, BrokerCredential, Portfolio, TokenPrincipal, User, LedgerTrade
+from app.domain.models import AuthToken, BrokerCredential, Portfolio, TokenPrincipal, User, LedgerEntry
 from app.main import create_app
 
 
@@ -356,7 +356,7 @@ def _trade_payload(**overrides) -> dict:
     return base
 
 
-def _make_ledger_trade(**kwargs) -> LedgerTrade:
+def _make_ledger_trade(**kwargs) -> LedgerEntry:
     defaults = dict(
         ledger_entry_id=1,
         portfolio_id="005930",
@@ -371,7 +371,7 @@ def _make_ledger_trade(**kwargs) -> LedgerTrade:
         recorded_at=datetime(2026, 5, 1, 1, 0, 0, tzinfo=timezone.utc),
     )
     defaults.update(kwargs)
-    return LedgerTrade(**defaults)
+    return LedgerEntry(**defaults)
 
 
 def _make_ledger_principal() -> TokenPrincipal:
@@ -381,7 +381,7 @@ def _make_ledger_principal() -> TokenPrincipal:
     )
 
 
-def _make_mock_repo_for_ledger(monkeypatch, trade: LedgerTrade, created: bool, found: bool = True) -> MagicMock:
+def _make_mock_repo_for_ledger(monkeypatch, trade: LedgerEntry, created: bool, found: bool = True) -> MagicMock:
     settings = _make_app_db(monkeypatch)
     mock_repo = MagicMock()
     mock_repo.get_token_principal.return_value = _make_ledger_principal()