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/domain/models.py

@@ -126,18 +126,31 @@ class Portfolio:
 @dataclass
 class LedgerEntry:
     portfolio_id: str
-    recorded_at: datetime
-    action: str
-    price: float
     quantity: float
-    cash_balance_after: float
-    asset_quantity_after: float
-    total_value_after: float
+    price: float
+    ledger_entry_id: int = 0
+    order_id: str | None = None
+    execution_id: str | None = None
+    side: str | None = None
     fee_amount: float = 0.0
     tax_amount: float = 0.0
+    executed_at: datetime | None = None
+    recorded_at: datetime | None = None
+    cash_balance_after: float = 0.0
+    asset_quantity_after: float = 0.0
+    total_value_after: float = 0.0
     profit_buffer_after: float = 0.0
     strategy_set_id: str = "legacy"
     note: str = ""
+    action: str | None = None
+
+    def __post_init__(self) -> None:
+        if self.side is None and self.action:
+            self.side = str(self.action).lower()
+        if self.action is None and self.side:
+            self.action = str(self.side).upper()
+        if self.executed_at is None:
+            self.executed_at = self.recorded_at
 
 
 @dataclass
@@ -170,17 +183,3 @@ class BrokerCredential:
     credential_version: int = 1
     last_rotated_at: datetime | None = None
 
-
-@dataclass
-class LedgerTrade:
-    ledger_entry_id: int
-    portfolio_id: str
-    order_id: str
-    execution_id: str
-    side: str
-    quantity: float
-    price: float
-    fee_amount: float
-    tax_amount: float
-    executed_at: datetime
-    recorded_at: datetime | None = None