feat: add project conversation pagination APIs

[JAVA-LW]

Summary

• add GET /api/conversation-projects for project workspace summaries
• add exact workspace filtering to the existing cursor-paginated conversation endpoint
• derive project membership from persisted extra.workspace, including legacy rows
• exclude backend-managed temporary workspaces, health checks, and team conversations

Closes #605

Frontend

Companion AionUi PR will consume this API to load the latest 5 conversations per project and fetch 5 more on demand.

Validation

• cargo fmt --all -- --check
• cargo check -p aionui-conversation -p aionui-db -p aionui-api-types
• project summary repository test
• exact workspace pagination repository test

[JAVA-LW]

Frontend companion PR: iOfficeAI/AionUi#3564

[IceyLiu]

aionui现在的存储不合理， 很难基于这个不合理的存储结构设计出来一个合理的分页逻辑。准备把存储逻辑改合理后，再做分页逻辑 @JAVA-LW