feat(platform): Implement legacy system integration with Wrapper Bridge architecture

Complete integration of the old clinical research platform (www.xunzhengyixue.com)
into the new AI platform via Token injection + iframe embedding:

Backend:
- Add legacy-bridge module (MySQL pool, auth service, routes)
- POST /api/v1/legacy/auth: JWT -> phone lookup -> Token injection into old MySQL
- Auto-create user in old system if not found (matched by phone number)

Frontend:
- LegacySystemPage: iframe container with Bridge URL construction
- ResearchManagement + StatisticalTools entry components
- Module registry updated from external links to iframe embed mode

ECS (token-bridge.html deployed to www.xunzhengyixue.com):
- Wrapper Bridge: sets cookies within same-origin context
- Storage Access API for cross-site dev environments
- CSS injection: hide old system nav/footer, remove padding gaps
- Inner iframe loads target page with full DOM access (same-origin)

Key technical decisions:
- Token injection (direct MySQL write) instead of calling login API
- Wrapper Bridge instead of parent-page cookie setting (cross-origin fix)
- Storage Access API + SameSite=None;Secure for third-party cookie handling
- User isolation guaranteed by phone number matching

Documentation:
- Integration plan v4.0 with full implementation record
- Implementation summary with 6 pitfalls documented
- System status guide updated (ST module now integrated)

Tested: Local E2E verified - auto login, research management, 126 statistical
tools, report generation, download, UI layout all working correctly

Made-with: Cursor

backend/package.json

@@ -50,6 +50,7 @@
     "jsonrepair": "^3.13.1",
     "jsonwebtoken": "^9.0.2",
     "jspdf": "^3.0.3",
+    "mysql2": "^3.18.2",
     "openai": "^6.16.0",
     "p-queue": "^9.0.1",
     "pg-boss": "^12.5.2",