docs(expense): add expense tracking implementation plan
18 TDD tasks: Ministry prerequisite (entity/seed/endpoint), expense category entities + seed, Expense + MonthlyStatement entities, EF migration, IFileStorage + local-disk impl, DTOs, three services with tests, controllers with auth split, and four Angular pages + nav/routes wiring. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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