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/schemas/v1.py

@@ -122,8 +122,8 @@ class LedgerTradeRequest(BaseModel):
 class LedgerTradeResponse(BaseModel):
     ledger_entry_id: int
     portfolio_id: str
-    order_id: str
-    execution_id: str
+    order_id: str | None
+    execution_id: str | None
     side: str
     quantity: float
     price: float
@@ -131,6 +131,8 @@ class LedgerTradeResponse(BaseModel):
     tax_amount: float
     executed_at: datetime
     recorded_at: datetime | None = None
+    cash_balance_after: float = 0.0
+    asset_quantity_after: float = 0.0
     status: Literal["new", "duplicate"] = "new"