perf: reuse SecureRandom instance in MockLnServer

[Copilot]

Addresses performance feedback from PR #57: SecureRandom instantiation on every invoice generation is expensive.

Changes

• MockLnServer: Made SecureRandom a static final field instead of creating new instances in generateValidBolt11Invoice()
• .gitignore: Fixed pattern to exclude all target/ directories, not just root-level

// Before: new instance on every call
private String generateValidBolt11Invoice() {
    SecureRandom random = new SecureRandom();
    // ...
}

// After: shared thread-safe instance
public class MockLnServer {
    private static final SecureRandom SECURE_RANDOM = new SecureRandom();
    
    private String generateValidBolt11Invoice() {
        // uses SECURE_RANDOM
    }
}

SecureRandom is thread-safe and initialization involves entropy gathering, making this a meaningful optimization for high-throughput scenarios.

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[github-actions]

Qodana Community for JVM

2 new problems were found

Inspection name	Severity	Problems
AutoCloseable used without 'try'-with-resources	🔶 Warning	1
Simplifiable conditional expression	🔶 Warning	1

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