docs(expense): add expense tracking & reimbursement design spec
Covers all five PLANNING §3.6d items: category seed (11 groups/38 subs),
vendor direct payment, staff reimbursement with receipt upload + self-service
submission, finance approval workflow (Draft→PendingApproval→Approved→Paid),
and monthly reconciliation statement. Per DB_SCHEMA §8.
Key decisions: IFileStorage abstraction + local-disk impl for receipts
(Azure Blob deferred), member self-submission alongside finance entry,
soft-delete Expense, cash-basis (Paid-only) monthly expense totals.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Chris Chen
fdd0d7c8e1