Add Transaction Tags/Labels Feature

[pyang4s]

Description:
Implement a tagging system to categorize transactions with multiple labels (e.g., "tax-deductible", "essential", "discretionary").

Acceptance Criteria:

• Create tags table (id, name, description)
• Create transaction_tags junction table for many-to-many relationships
• Update schema.sql with new tables
• Add sample tags to sample_data.sql
• Create notebook query to filter transactions by tag
• Update database schema documentation in README

Estimated Effort: 3-4 hours