feat: make persistence composit + apply for mint invoices

[jafar75]

Summary

Implements composite persistence for MintInvoiceManager using the child() method to split large monolithic JSON files into smaller, organized files.

Changes

• Added child(key) method to Persistence trait
• Implemented child() for JsonFilePersistence and InMemoryPersistence
• Updated MintInvoiceManager to store metadata in root JSON and full invoice data (lots, positions) in individual files
• File structure: chain_{id}/address_{addr}/order_{id}.json
• As discussed, invoice metadata (IDs, amounts, timestamps) is stored in the root JSON file, while large nested data (lots array and position object) is stored in separate child files. This keeps the root file lightweight for fast queries while heavy data is loaded on-demand only when needed.

Migration

Run one-time migration test (commented) to convert existing data from old format to new format with child files.

[sadhbh-c0d3]

removed persistence folder :)

[sadhbh-c0d3]

Looks all great!