[FIN-03] Implement Transaction Recording

[kilip]

Description

Implement income and expense transaction recording.

Subtasks

• FIN-03-ST-01 Implement service-layer period lock enforcement in TransactionService
• FIN-03-ST-02 Implement TransactionService.create(), update(), delete() with lock check
• FIN-03-ST-03 Build transaction create form at /finance/[year]/[month]/new
• FIN-03-ST-04 Build transaction detail/edit page at /finance/[year]/[month]/[transactionId]
• FIN-03-ST-05 Optional receipt photo upload to Google Drive

Acceptance Criteria

• FIN-03-AC-01 Only treasurer can record transactions
• FIN-03-AC-02 Transactions on a locked period are rejected with a clear error
• FIN-03-AC-03 amount, categoryId, and date are required fields
• FIN-03-AC-04 Receipt photo upload is optional

Technical Notes

• Docs: PRD#63-parish-financial-recording · TDD#48-financial-period-locking-enforcement
• Screen spec: docs/screen.md → Transaction Create, Transaction Detail / Edit