Remove PostgreSQL related code from App Proxy

## Goal

Complete removal of PostgreSQL-related code

## Files to Remove

```
src/ai/backend/appproxy/coordinator/models/
├── worker.py
├── circuit.py
├── endpoint.py
├── token.py
├── base.py
├── utils.py
└── alembic/ # Entire directory

src/ai/backend/appproxy/coordinator/
├── pglock.py
├── dependencies/infrastructure/database.py
├── health/database.py
├── cli/dbschema.py
```

## Config to Remove

- `alembic-appproxy.ini`
- `DBConfig` from `coordinator/config.py`

JIRA Issue: BA-3228