Add Data Validation Script

[pyang4s]

Description:
Create a validation script to check data integrity and identify anomalies in the database.

Acceptance Criteria:

• Create scripts/validate.sh
• Check for negative amounts in transactions
• Check for future-dated transactions
• Identify duplicate transactions
• Find transactions without descriptions
• Output validation report to console
• Add usage instructions to README

Estimated Effort: 2-3 hours