test: 程式碼.js branch gaps + re-ratchet thresholds to 95/90/95/95 (#107 P1)

[cheerc]

What

Fill backend branch coverage gaps in 程式碼.js and re-ratchet coverage thresholds. Part of #107 Phase 2 P1.

Part 1: Branch Gap Fill

Added 2 new test cases in backend.test.js:

Gap	Line	Test	Description
L458	copySchedule	Source ID not in Data index	_findScheduleRowInfo returns -1 for nonexistent source
L542-543	getVersionData	Empty history sheet	lastRow < 2 (header only, no data rows)

Part 2: Threshold Re-Ratchet

Metric	Old	New	Actual
Statements	67%	95%	98.55%
Branches	74%	90%	94.09%
Functions	82%	95%	98.93%
Lines	67%	95%	99.45%

Changed Files (2)

• vitest.config.js (threshold re-ratchet)
• tests/unit/backend.test.js (+2 tests)

Verification

• npm test: 1156 tests passed (37 files) ✅
• npx vitest run --coverage: all thresholds pass ✅

refs #107

[cheerc]

✅ VERIFIED

Reviewed HEAD: 465076c
Reviewer: cb-team-reviewer
Audit mode: fast_path (D1 — test-only)

Files Reviewed (2)

• vitest.config.js — threshold re-ratchet 67/74/82/67 → 95/90/95/95
• tests/unit/backend.test.js (+32) — 2 branch gap tests

Fast Path Checklist

• No production code changes
• Threshold re-ratchet: ~3-5% buffer from actuals (~98/93/98/99) ✓
• getVersionData empty history (L542-543): correct error assertion 找不到 ✓
• copySchedule source not found (L458): correct error assertion 找不到來源課表 ✓
• refs [Test-Coverage] Phase 2 — 覆蓋缺口補齊 + 重構前置強化 #107

Evidence

• cited: vitest.config.js — lines:95, functions:95, branches:90, statements:95
• cited: backend.test.js:145-155 — getVersionData empty history test
• cited: backend.test.js:273-293 — copySchedule not-found test