feat(iit): Initialize IIT Manager Agent MVP - Day 1 foundation complete

Summary:
- Launch IIT Manager Agent (AI-driven IIT research assistant)
- Complete Day 1/14: Database schema, module structure, WeChat integration

Database Layer:
- Add iit_schema with 5 tables
- Include V1.1 fields: cachedRules, lastSyncAt, miniProgramOpenId
- All CRUD tests passed

Module Structure:
- Create backend/src/modules/iit-manager/
- 223 lines TypeScript types
- Health check endpoint working

WeChat Integration:
- App registered: CorpID ww6ab493470ab4f377
- Access Token verified successfully

Documentation:
- Update system status doc v2.3 -> v2.4
- Complete IIT doc structure
- Technical plan V1.1 (2170 lines)
- MVP task list (615 lines)

Status: Day 1 complete (11/11 tasks), ready for Day 2