fix(executor): keep strong references to fire-and-forget Telegram notification tasks

asyncio.create_task() results were discarded, and the event loop only holds
a weak reference to running tasks — a pending notification could be
garbage-collected before executing, silently dropping Telegram messages
(documented asyncio pitfall).

Route the three notification call sites (trade_opened, trade_legacy_closed,
trade_closed) through _notify_in_background(), which stores the task in a
module-level set and discards it on completion. Notifications stay
fire-and-forget; no business logic changed.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bot/executor/paper.py

@@ -22,6 +22,19 @@ log = logging.getLogger(__name__)
 # NOTE: this is a heuristic — see COMMISSION_RATE in bayesian.py for context.
 POLYMARKET_FEE = 0.02  # 2%
 
+# Strong references to in-flight notification tasks.  The event loop only
+# keeps a weak reference to tasks created via create_task(), so without this
+# set a pending Telegram notification could be garbage-collected before it
+# runs.  Tasks remove themselves from the set on completion.
+_background_tasks: set[asyncio.Task] = set()
+
+
+def _notify_in_background(coro) -> None:
+    """Fire-and-forget a Telegram notification, keeping the task referenced."""
+    task = asyncio.create_task(coro)
+    _background_tasks.add(task)
+    task.add_done_callback(_background_tasks.discard)
+
 
 @dataclass
 class Trade:
@@ -205,7 +218,7 @@ class PaperExecutor:
         # Persist to DB
         await self._db.save_trade(trade)
 
-        asyncio.create_task(
+        _notify_in_background(
             telegram.trade_opened(trade.question, trade.direction, trade.size_usdc, trade.edge_net)
         )
 
@@ -226,7 +239,7 @@ class PaperExecutor:
             "LEGACY_CLOSE market=%s | returned $%.2f to cash | %s",
             market_id, cost, reason[:80],
         )
-        asyncio.create_task(
+        _notify_in_background(
             telegram.trade_legacy_closed(question or market_id, cost, reason)
         )
         return cost
@@ -281,7 +294,7 @@ class PaperExecutor:
             "Closed position in %s, resolution=%.1f payout=$%.2f pnl=%+.2f",
             market_id, resolution, payout, pnl,
         )
-        asyncio.create_task(
+        _notify_in_background(
             telegram.trade_closed(question or market_id, pnl)
         )
         return pnl