test: add unit/integration/E2E test suites, fix create_session bug, update docs

- Unit tests: test_session.py (27), test_error_kb.py (24), test_agent.py hardened
- Integration tests: test_api_integration.py (25) with FastAPI TestClient
- E2E tests: main-flows.spec.ts (8) with Playwright + API mocking
- Bug fix: backend/session.py create_session() missing session_id parameter
- Config: frontend/playwright.config.ts, npm run test:e2e
- Docs: update CLAUDE.md v9, .gitignore for test artifacts/eval reports

.gitignore

@@ -13,6 +13,19 @@ db/chroma/
 sessions/
 logs/
 db/
+# 自动评测 (Mavis AI)
+.mavis/
+EVALUATION_REPORT.md
+
+# 上传文件
+uploads/
+
+# OCR 临时输出
+ocr_raw_positions.json
+
+# Playwright E2E 测试产物
+frontend/test-results/
+
 # RAG 管线中间产物 (rag 子模块内)
 rag/jrxml_chunker_output/
 rag/embeddings/