Fix incorrect SQLite database path in market app

[TanmayGaidhani]

Fixes #5

Problem

The /market route raised the following error:

sqlite3.OperationalError: no such table: item

Cause

The SQLite database path was resolving to the instance directory,
where the required item table was not present.

Fix

Updated the SQLite database path to explicitly use market.db
from the application root directory.

Testing

• Reproduced the /market 500 error locally
• Confirmed instance/market.db did not contain the item table
• Confirmed the project market.db contains the item table
• Updated the database path
• Verified that the /market route works successfully