feat: show max-hold session countdown

app/schemas/paper_trade.py

@@ -53,3 +53,7 @@ class PaperTradeResponse(BaseModel):
     # when the trailing exit policy is active.
     trailing_stop: float | None = None
     trailing_distance_pct: float | None = None
+    # Trading sessions represented by post-entry OHLCV bars. These are populated
+    # only while the active exit policy has a max-hold rule.
+    sessions_held: int | None = None
+    sessions_remaining: int | None = None