Relations support (belongs_to, has_many)

[RAprogramm]

Problem

Entities exist in isolation. Real data models have relationships:

• User has many Posts
• Post belongs to User
• Order has many OrderItems

Currently users must write all join logic manually.

Why it matters

• Every real project needs relations
• Manual joins are error-prone
• This is the ci(deps): bump actions/upload-artifact from 4 to 6 #1 missing feature for adoption
• Differentiates from simple DTO generators

Acceptance criteria

• Declare relations via attributes
• Generate join methods in repository
• Type-safe relation traversal
• No eager loading magic (explicit only)

Scope limitation

First version: simple belongs_to/has_many without nested loading.