Add process_due_subscriptions coverage

[awyoo]

/claim #332

Summary

• Add a focused subscription_test.rs module for process_due_subscriptions.
• Cover the authorized operator path returning 0 when no subscriptions are due.
• Cover non-operator rejection with Error::Unauthorized.
• Document the Implement process_due_subscriptions with active subscription index #302 follow-up for full due-subscription processing coverage.

Verification

• git diff --check
• Not run: cargo test -p fluxapay process_due_subscriptions -- --nocapture because this local environment does not have cargo/rustc installed.

Bounty

Payout address: 0xe80506A1431B3B4aAca8B260838481deBbdF75Ab