Add connection pooling for PostgreSQL

## Description
Implement connection pooling to manage database connections efficiently and improve performance.

## Requirements
- [ ] Integrate with connection pooling library (deadpool-postgres or bb8)
- [ ] Support configurable pool sizes
- [ ] Add connection health checks
- [ ] Implement connection recycling
- [ ] Add metrics and monitoring
- [ ] Write comprehensive tests

## Acceptance Criteria
- Automatic connection pool management
- Configurable pool parameters (min/max connections, timeouts)
- Proper connection cleanup and recycling
- Performance monitoring capabilities

## Files to modify
- `beatrix_core/Cargo.toml`
- `beatrix_core/src/relational/db/postgres.rs`
- `beatrix_core/src/relational/pool.rs` (new file)

## Example Usage
```rust
let pool = PostgresPool::new("postgres://...", PoolConfig::default()).await?;
let db = pool.get().await?;
let users = User::select().fetch_all(&db).await?;
```