Design Redis data model for App Proxy state

## Goal

Convert PostgreSQL tables to Redis data structures

## Work Items

- Define Worker data structure (Hash + TTL)
- Define Circuit/Route data structure (reuse existing `endpoint.{id}.route_connection_info`)
- Define Token data structure (Hash + TTL)

## Redis Key Design

```
appproxy:worker:{worker_id} # Worker info (Hash)
appproxy:worker:{worker_id}:heartbeat # Heartbeat (TTL: 60s)
appproxy:worker:{worker_id}:ports # Occupied ports (Set)
appproxy:worker:all # All worker IDs (Set)
appproxy:token:{token_id} # Token info (Hash, TTL: 1h)

# Existing keys (pushed by Manager)
endpoint.{endpoint_id}.route_connection_info
endpoint.{endpoint_id}.health_check_enabled
endpoint.{endpoint_id}.health_check_config
```

## Files to Modify

- `src/ai/backend/appproxy/common/types.py` - Add Redis key constants

JIRA Issue: BA-3222