Add TTL-based entry expiry to the invoice cache

[Kingsman-99]

Description

src/cache.ts stores invoice data indefinitely with no expiry. Once an entry is written it stays forever, causing stale data to be served long after an invoice changes state on-chain. This issue adds TTL (time-to-live) support so entries are automatically evicted after a configurable duration.

Acceptance Criteria

• Cache class accepts an optional ttlMs: number constructor argument (default: no expiry — backward compatible)
• Each set() records the write timestamp alongside the value
• get() returns undefined and deletes the entry if Date.now() - writeTime > ttlMs
• has() also respects TTL — returns false for expired entries
• A new purgeExpired() method removes all expired entries in one sweep
• Existing behaviour unchanged when ttlMs is not set
• Unit tests: entry expires after TTL, entry valid before TTL, purgeExpired removes only expired entries, has() returns false for expired

Context

• Target file: src/cache.ts
• Do not add any new dependencies — use Date.now() only
• All existing tests in src/__tests__/ must continue to pass

[Peculiarr7]

Hi! I came across this issue and would love to work on it. I have experience with similar tasks and I'm confident I can investigate, implement a clean solution, and thoroughly test it before submitting a PR. If the issue is still open, I would appreciate it if you could assign it to me. I’ll prioritize this immediately and ensure a reliable solution is delivered as soon as possible. Thank you!

[bobo-24]

Hi! I’m a full-stack developer with a strong blockchain background, and I’m confident I can resolve this issue quickly and effectively. I’ll deliver a clean, reliable solution with immediate attention to detail, ensuring it’s completed as soon as possible. Please assign it to me—I’m ready to get started right away.

[yahia008]

hi team kindly assign me i can work on dis issue

[grantfox-oss]

🦊 GrantFox — @yahia008 has been assigned to this issue as part of the Third Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #606)
2. Your PR will be reviewed by the Stellar-split maintainers

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