fix(api): Prevent duplicate notifications for existing transactions during sync.

The notification system was incorrectly sending notifications for existing
transactions that were being updated during sync operations. This change
modifies the transaction persistence logic to only return genuinely new
transactions, preventing duplicate notifications while maintaining data
integrity through INSERT OR REPLACE.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

leggen/services/database_service.py

@@ -693,7 +693,7 @@ class DatabaseService:
 
             # Add the display_name column
             cursor.execute("""
-                ALTER TABLE accounts 
+                ALTER TABLE accounts
                 ADD COLUMN display_name TEXT
             """)
 
@@ -857,6 +857,14 @@ class DatabaseService:
 
             for transaction in transactions:
                 try:
+                    # Check if transaction already exists before insertion
+                    cursor.execute(
+                        """SELECT COUNT(*) FROM transactions
+                           WHERE accountId = ? AND transactionId = ?""",
+                        (transaction["accountId"], transaction["transactionId"]),
+                    )
+                    exists = cursor.fetchone()[0] > 0
+
                     cursor.execute(
                         insert_sql,
                         (
@@ -873,7 +881,11 @@ class DatabaseService:
                             json.dumps(transaction["rawTransaction"]),
                         ),
                     )
-                    new_transactions.append(transaction)
+
+                    # Only add to new_transactions if it didn't exist before
+                    if not exists:
+                        new_transactions.append(transaction)
+
                 except sqlite3.IntegrityError as e:
                     logger.warning(
                         f"Failed to insert transaction {transaction.get('transactionId')}: {e}"