fix: standardize exchange database path to use centralized data directory with environment variable

🔧 Database Path Standardization:
• Change DATABASE_URL environment variable to EXCHANGE_DATABASE_URL
• Update default database path from ./exchange.db to /var/lib/aitbc/data/exchange/exchange.db
• Apply consistent path resolution across all exchange database connections
• Update database.py, seed_market.py, and simple_exchange_api.py with new path
• Maintain backward compatibility through

apps/exchange/scripts/seed_market.py

@@ -7,7 +7,9 @@ from datetime import datetime
 def seed_initial_price():
     """Create initial trades to establish market price"""
     
-    conn = sqlite3.connect('exchange.db')
+    import os
+    db_path = os.getenv("EXCHANGE_DATABASE_URL", "sqlite:////var/lib/aitbc/data/exchange/exchange.db").replace("sqlite:///", "")
+    conn = sqlite3.connect(db_path)
     cursor = conn.cursor()
     
     # Create some initial trades at different price points