Build creator-configurable auto-resolve fallback chain

[Kingsman-99]

Label: complexity: high
Points: 200

Description

auto_resolve_rules (existing) evaluates rules but there's no documented behavior for what happens if none match — currently it's effectively a no-op. This issue adds an explicit fallback action when no rule's threshold is met by the deadline.

Technical Context

Involves types.rs (fallback_action: Option<ResolveAction> on InvoiceOptions/InvoiceExt), lib.rs auto_resolve() — after iterating auto_resolve_rules and finding no match, execute fallback_action if configured instead of leaving the invoice in limbo.

Acceptance Criteria

• When no auto_resolve_rules entry matches and fallback_action is set, that action executes (release or refund)
• When no fallback_action is configured, behavior is unchanged from today (no-op, documented as intentional)
• auto_resolve() is idempotent — calling it again after resolution doesn't re-trigger
• Test: invoice with rules that don't match and a Refund fallback correctly refunds via auto_resolve()
• All existing cargo tests pass
• cargo clippy passes with zero warnings

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[Max-Owolabi]

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[johnsaviour56-ship-it]

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@johnsaviour56-ship-it's PR #253 was approved and merged by @Kingsman-99.

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