Add project summaries and workspace-filtered conversation pagination

## Problem

The AionUi sidebar needs two-step conversation loading: first list projects, then cursor-page conversations inside each project. The existing conversation endpoint can page globally but cannot provide project summaries or filter an exact workspace.

## Proposed API

- `GET /api/conversation-projects`
- Return custom project workspace paths, conversation counts, and latest activity.
- Exclude backend-managed temporary workspaces, health-check conversations, and team conversations.
- Remain compatible with historical rows that only persist `extra.workspace`.
- Extend `GET /api/conversations` with an exact `workspace` filter.
- Reuse the existing cursor and `limit` response contract.

## Consumer

The corresponding AionUi change loads 5 conversations per project and fetches 5 more on demand.

Start by reading the existing GET /api/conversations implementation and its cursor and limit response contract. Define GET /api/conversation-projects around the stated exclusions and historical extra.workspace compatibility, then verify exact workspace filtering and the two-step loading behavior described for AionUi.

Written by the indexing model from the issue text.