feat(auto-resolve): add configurable fallback action for unmatched auto-resolve rules

[johnsaviour56-ship-it]

Summary

This PR adds support for configurable fallback actions in auto_resolve(), ensuring invoices do not remain unresolved when no auto-resolve rule matches at the configured deadline.

Changes

• Added an optional fallback_action field to invoice options and persisted invoice data.
• Updated auto_resolve() to execute the configured fallback action (release or refund) when no auto-resolve rule matches.
• Preserved existing behavior when no fallback action is configured, leaving the invoice unchanged.
• Ensured auto_resolve() remains idempotent, preventing repeated execution after an invoice has already been resolved.
• Added tests covering fallback refund behavior, idempotency, and the no-op path when no fallback is configured.

Testing

Added coverage for:

• Fallback action execution when no rules match
• Refund fallback via auto_resolve()
• No-op behavior when no fallback is configured
• Idempotent repeated calls to auto_resolve()
• Regression checks for existing auto-resolve functionality

Acceptance Criteria

• Executes configured fallback action when no rules match
• Preserves existing no-op behavior when no fallback is configured
• auto_resolve() remains idempotent
• Added tests for fallback refund scenario
• All existing tests pass
• cargo clippy passes without warnings

Closes #224

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