Database indexing optimization for financial queries

[rohitdash08]

Improve performance of high-frequency queries.

Acceptance Criteria

• Production ready implementation
• Includes tests
• Documentation updated

[a827681306]

/attempt #128

I'll implement this with full test coverage and documentation. PR incoming shortly.

[zhaog100]

/attempt #128

Implementing database indexing optimization:

• Composite indexes for common query patterns
• Partial indexes for expense queries
• EXPLAIN analysis for slow queries
• Migration script with new indexes

Starting immediately.