Add DELETE operation support for PostgreSQL

## Description
Implement DELETE functionality to allow removing records from PostgreSQL.

## Requirements
- [ ] Implement DELETE query builder
- [ ] Support WHERE clauses for targeted deletes
- [ ] Add soft delete option
- [ ] Implement cascade delete handling
- [ ] Write comprehensive tests

## Acceptance Criteria
- User can call `User::delete().where_clause(User::id().eq(1)).execute(&db)`
- Support for conditional deletes
- Return affected row count
- Safe deletion with proper constraints

## Files to modify
- `beatrix_core/src/relational/delete.rs` (new file)
- `beatrix_core/src/relational/mod.rs`

## Example Usage
```rust
let affected = User::delete()
.where_clause(User::id().eq(1))
.execute(&db).await?;
```