Add soft delete support

[quinnjr]

Description

Implement soft delete functionality with automatic filtering of deleted records.

Tasks

• Add @softDelete schema attribute
• Auto-filter deleted records on queries
• Add withDeleted() to include soft-deleted
• Add onlyDeleted() to query only deleted
• Support custom deleted_at column name
• Add restore() operation

Schema Example

model User {
    id        Int       @id @auto
    email     String
    deletedAt DateTime? @softDelete
}

Query Examples

// Automatically excludes soft-deleted
let users = client.user().find_many().exec().await?;

// Include soft-deleted
let all = client.user().find_many().with_deleted().exec().await?;

// Only deleted
let deleted = client.user().find_many().only_deleted().exec().await?;

// Restore
client.user().restore(user::id::equals(1)).exec().await?;