[FIN-04] Implement Financial Report

[kilip]

Description

Implement the financial report with yearly overview and drill-down capability.

Subtasks

• FIN-04-ST-01 Build yearly overview at /finance
• FIN-04-ST-02 Build monthly detail at /finance/[year]/[month]
• FIN-04-ST-03 UserRole gate: only treasurer, pastor, and executive-board can access
• FIN-04-ST-04 Display income/expense breakdown clearly

Acceptance Criteria

• FIN-04-AC-01 Yearly overview shows monthly income/expense summary
• FIN-04-AC-02 Drill-down to monthly detail works
• FIN-04-AC-03 Drill-down to individual transaction detail works
• FIN-04-AC-04 Unauthorized roles receive 403

Technical Notes

• Docs: PRD#64-financial-reports · TDD#83-permission-rules
• Screen spec: docs/screen.md → Financial Report, Monthly Period Detail