fix: correct Database default path to match system-wide db_path

Changes:
- Change Database.__init__ default from "data/trading.db" to "data/jobs.db"

Root Cause:
- Job creation uses "data/jobs.db" (via JobManager, SimulationWorker)
- BaseAgent's Database() was using "data/trading.db" by default
- This caused jobs table to exist in jobs.db but trading_days INSERT
  tried to reference job_id from trading.db, causing FK constraint failure

Impact:
- Fixes: "FOREIGN KEY constraint failed" when creating trading_day records
- Ensures all components use same database file for referential integrity
- Maintains DEV/PROD mode isolation via get_db_path()

Related: api/database.py:521

api/database.py

@@ -518,7 +518,7 @@ class Database:
         """
         if db_path is None:
             from tools.deployment_config import get_db_path
-            db_path = get_db_path("data/trading.db")
+            db_path = get_db_path("data/jobs.db")
 
         self.db_path = db_path
         self.connection = sqlite3.connect(db_path, check_same_thread=False)