fix: use 'no_trade' action type for initial position

Changed action_type from 'init' to 'no_trade' in _initialize_starting_position()
to comply with database CHECK constraint that only allows 'buy', 'sell', or 'no_trade'.

Fixes sqlite3.IntegrityError during position initialization.

api/model_day_executor.py

@@ -334,7 +334,7 @@ class ModelDayExecutor:
             )
             VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)
         """, (
-            self.job_id, self.date, self.model_sig, 0, "init",
+            self.job_id, self.date, self.model_sig, 0, "no_trade",
             initial_cash, initial_cash, session_id, created_at
         ))