docs(04-04): complete Expense Reports and Audit Trail plan

Tasks completed: 2/2
- ExpenseReportService and AuditTrailService with API routes
- 9 passing integration tests

SUMMARY: .planning/phases/04-inventory-expenses-and-financial-reports/04-04-SUMMARY.md

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.planning/phases/04-inventory-expenses-and-financial-reports/04-04-SUMMARY.md

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+---
+phase: 04-inventory-expenses-and-financial-reports
+plan: 04
+subsystem: reports, accounting
+tags: [expense-reports, audit-trail, journal-entries, ACCT-08, prisma]
+
+# Dependency graph
+requires:
+  - phase: 04-03
+    provides: ExpenseService, VendorService, expense categories, vendor model
+  - phase: 02-02
+    provides: JournalEntryService, referenceType/referenceId on JEs
+provides:
+  - ExpenseReportService with category/vendor/summary aggregation
+  - AuditTrailService with JE audit trail (ACCT-08 compliance)
+  - Expense report API routes
+  - JE audit trail API route
+affects: []
+
+# Tech tracking
+tech-stack:
+  added: []
+  patterns:
+    - "Expense aggregation in JS from Prisma findMany (same pattern as outstanding reports)"
+    - "AuditTrailService works across all JE sources via referenceType/referenceId"
+
+key-files:
+  created:
+    - src/lib/services/expense-report-service.ts
+    - src/lib/services/audit-trail-service.ts
+    - src/app/api/reports/expenses/route.ts
+    - src/app/api/reports/expenses/by-vendor/route.ts
+    - src/app/api/accounting/journal-entries/[id]/audit/route.ts
+    - src/lib/__tests__/expense-report-service.test.ts
+  modified: []
+
+key-decisions:
+  - "Expense aggregation done in JS after findMany (same pattern as outstanding reports, collector balances)"
+  - "AuditTrailService queries JEs by referenceType+referenceId -- works for all sources (Invoice, Payment, Expense, Collection, etc.)"
+  - "No Vendor bucket uses null vendorId key in grouping -- expenses without vendor are still reported"
+
+patterns-established:
+  - "AuditTrailService pattern: getJournalEntryAudit for single JE, getAuditTrailForEntity for all JEs related to a source record"
+
+# Metrics
+duration: 20min
+completed: 2026-03-05
+---
+
+# Phase 4 Plan 4: Expense Reports and Audit Trail Summary
+
+**ExpenseReportService aggregates spending by category/vendor with date filtering; AuditTrailService traces every JE to its creator and source transaction (ACCT-08)**
+
+## Performance
+
+- **Duration:** 20 min
+- **Started:** 2026-03-05T06:35:27Z
+- **Completed:** 2026-03-05T06:55:12Z
+- **Tasks:** 2
+- **Files created:** 6
+
+## Accomplishments
+- Expense reports by category and vendor with date range filtering (only POSTED expenses)
+- Full JE audit trail showing creator, timestamp, source reference, and line details
+- Audit trail works across ALL JE sources (proven with Payment JE test)
+- 9 passing integration tests covering reports, filtering, and cross-source audit
+
+## Task Commits
+
+Each task was committed atomically:
+
+1. **Task 1: ExpenseReportService and AuditTrailService** - `6bc24a3` (feat)
+2. **Task 2: Tests for expense reports and audit trail** - `787b22e` (test)
+
+## Files Created/Modified
+- `src/lib/services/expense-report-service.ts` - Expense aggregation by category, vendor, and combined summary
+- `src/lib/services/audit-trail-service.ts` - JE audit trail with creator, source reference, approval info
+- `src/app/api/reports/expenses/route.ts` - GET expense summary by category (ADMIN)
+- `src/app/api/reports/expenses/by-vendor/route.ts` - GET expense summary by vendor (ADMIN)
+- `src/app/api/accounting/journal-entries/[id]/audit/route.ts` - GET JE audit trail (ADMIN, OFFICE_STAFF)
+- `src/lib/__tests__/expense-report-service.test.ts` - 9 integration tests
+
+## Decisions Made
+- Expense aggregation done in JS after findMany -- same pattern as outstanding reports and collector balances (acceptable for ISP scale)
+- AuditTrailService queries by referenceType+referenceId, works for all JE sources without source-specific logic
+- "No Vendor" bucket included in vendor reports for expenses without vendorId
+- Decimal amounts serialized as strings in API responses (consistent with other report endpoints)
+
+## Deviations from Plan
+
+None -- plan executed exactly as written.
+
+## Issues Encountered
+- Plan specified `npx jest` for test verification but project uses Vitest -- used `npx vitest run` instead (not a code deviation, just a verification command difference)
+
+## User Setup Required
+
+None -- no external service configuration required.
+
+## Next Phase Readiness
+- Phase 4 is now fully complete (all 5 plans: 04-01 through 04-05)
+- All financial reporting capabilities built: trial balance, income statement, balance sheet, expense reports, and audit trail
+- Ready for Phase 5 (Portal and Dashboard)
+
+---
+*Phase: 04-inventory-expenses-and-financial-reports*
+*Completed: 2026-03-05*