test: add missing backend unit tests to improve coverage from 87% to 99%

[devin-ai-integration]

Summary

Adds 22 new Jest unit tests across 4 backend test files to cover previously untested code paths. No production code changes.

Coverage improvement: 87.17% → 99.56% statements, 100% functions.

File	Area covered
reports.test.js	PDF generation success paths (with entries, empty, null descriptions, page breaks, separator lines), CSV download success path with file cleanup
clients.test.js	Department/email field updates (including empty → null), bulk DELETE /api/clients, create with all fields, try-catch error paths
workEntries.test.js	Unexpected error handling in POST/PUT try-catch blocks
database/init.test.js	closeDatabase() edge cases: already-closed, null db, concurrent close race condition

Remaining uncovered: Lines 129, 134 in reports.js (inside res.download callback) — these are covered via a custom Express app that mocks res.download rather than exercising the real download path, so Jest may not attribute full line coverage.

Review & Testing Checklist for Human

• CSV download tests use a custom Express app with mocked res.download (reports.test.js lines ~668-710, ~729-750). Verify this workaround actually exercises the intended code path in routes/reports.js lines 127-134, not just the mock. Consider whether the jest.mock call inside the test body behaves correctly with Jest's hoisting.
• PDFDocument mock pattern is duplicated across 6+ tests — verify the pipe/end stream simulation actually triggers the same code flow as real pdfkit (i.e., that doc.end() ending the response stream is a valid simulation).
• Bulk delete tests use function() syntax with this.changes to simulate SQLite3's callback context binding. Confirm this matches the actual sqlite3 API contract used in routes/clients.js.
• Run npm test -- --coverage in backend/ and confirm all 183 tests pass and no regressions in existing tests.

Notes

• The isClosing concurrent-close test (init.test.js line ~179) relies on timing — it starts a close, then starts a second close before completing the first. This exercises the polling/wait loop but may be fragile if internal implementation changes.

Link to Devin session: https://partner-workshops.devinenterprise.com/sessions/2ef8e7643cf74f5cb3876e0bd502d186

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 4 additional findings.