fix: fail closed without vss auth

[ben-kaufman]

This PR fails closed when LNURL-auth is missing so wallet backups cannot fall back to unauthenticated VSS.

Description

Removes the unauthenticated VSS client fallback. If LNURL-auth is not configured, backup setup and Lightning node build now error instead of using a public encryption key or fixed-header VSS.

Env currently always sets an LNURL-auth URL, so this is a footgun fix rather than a user-visible change on production configs.

Related: synonymdev/audit#55 (BITKIT-SEC-003). Library-side rejection is in synonymdev/vss-rust-client-ffi.

Linked Issues/Tasks

• https://github.com/synonymdev/audit/issues/55

Screenshot / Video

N/A

QA Notes

Manual Tests

• 1. Fresh wallet → backup/Lightning start with current Env LNURL-auth URL: setup still succeeds.
• 2. regression: empty LNURL-auth URL → VSS setup and node build fail instead of using unauthenticated VSS.

Automated Checks

• Added ChannelMigrationPersistenceTests coverage for retaining migration data on setup failure, clearing it after success, and preserving a newer pending migration.
• SwiftFormat and translation validation pass locally.

Made with Cursor

[greptile-apps]

Greptile Summary

The PR removes unauthenticated VSS fallback behavior so backup and Lightning-node initialization fail closed when LNURL-auth is unavailable.

• Requires LNURL-auth parameters when initializing the backup VSS client.
• Requires a non-empty LNURL-auth URL before building the Lightning node and during stale-monitor recovery.
• Adds a shared vssAuthRequired service error and documents the security change.

Confidence Score: 5/5

The PR appears safe to merge and consistently enforces fail-closed VSS authentication across the changed setup paths.

The backup client and Lightning builder now reject missing LNURL-auth rather than selecting unauthenticated construction, while preserving authenticated setup and stale-monitor recovery behavior.

Important Files Changed

Filename	Overview
Bitkit/Services/LightningService.swift	Removes fixed-header VSS node construction and rejects empty LNURL-auth configuration before either normal or stale-monitor-recovery builds.
Bitkit/Services/VssBackupClient.swift	Removes unauthenticated backup-client fallback and consistently reports the shared authentication-required error.
Bitkit/Utilities/Errors.swift	Adds localized handling for the new vssAuthRequired service error.
changelog.d/next/fail-closed.security.md	Accurately records that wallet backups no longer fall back to unauthenticated VSS.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A[VSS or Lightning setup] --> B{LNURL-auth configured?}
  B -- No --> C[Throw vssAuthRequired]
  B -- Yes --> D[Build authenticated VSS client or node]
  D --> E{Build succeeds?}
  E -- Yes --> F[Setup complete]
  E -- DangerousValue --> G[Enable stale monitor recovery]
  G --> H[Retry authenticated node build]
  E -- Other error --> I[Propagate setup failure]

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_{Reviews (1): Last reviewed commit: "fix: fail closed without vss auth" | Re-trigger Greptile}

[jvsena42]

Self-contained PR, no issues found

[jvsena42]

@ben-kaufman conflict