Add storage optimization for large batch records in bulk_payment

[Wilfred007]

What

Bulk_payment stores all batch records in instance storage which has a 64KB limit. Each BatchRecord is 200+ bytes, so after ~300 batches the contract hits limits.

Why

The contract becomes permanently unusable after enough batches. Cross_asset_payment correctly uses persistent storage.

Scope

In scope: Move batch records to persistent storage, add TTL for old records, storage usage query
Out of scope: Off-chain storage

Acceptance Criteria

• Batch records in persistent storage (not instance)
• Old records have TTL for cleanup
• Storage usage queryable
• No 64KB limit issue

Technical Context

• contracts/bulk_payment/src/lib.rs lines 152, 244 — instance storage
• contracts/cross_asset_payment/src/lib.rs line 172 — persistent storage (correct)