feat: add lnurl-pay callback retry fixture

[ovitrif]

Description

• Add an optional LNURL-pay profile with stable metadata and an invoice callback that keeps returning an error until explicitly switched healthy. Healthy responses contain fresh signed regtest invoices bound to the metadata, with exact millisatoshi amounts.
• Checked: Built the ARM64 Docker image and started an isolated Compose project to healthy. The HTTP contract test passed with one test and zero failures. Live HTTP probes verified repeated callback errors, the healthy switch, signed regtest invoice amount and metadata hash, remapped callback URLs and error reset. The lane check recognized the profile and the refreshed inventory offers the capability.

Out of Scope

• Lightning payment settlement and channel setup; the fixture generates signed invoices for callback fetching and decoding.

[ovi-reviewer]

Suggestion: 👍 Approve

The test run for synonymdev/bitkit-ios#867 at head 16f0966 used this change. The one item that needed it, J2, ran with the LNURL-pay retry fixture and passed.

The environment ran the change at commit 9f5ea99.

[ovi-reviewer]

Suggestion: 👍 Approve

The test run for synonymdev/bitkit-ios#867 at head e0e44a4 needed this change, and the one item that depended on it, J2, ran and passed.

The environment ran commit 9f5ea99 of this change, which adds the LNURL-pay profile with a callback that keeps returning an error until it is switched healthy. The fixture worked as intended for the run.