fix: remove no-op clearCache and fix cachedAt typing in claimable service

[wheval]

📌 Summary

Removes the misleading no-op clearCache() method from ClaimableAmountService and fixes the type hole on cache-hit responses by declaring cachedAt on ClaimableAmountResult.

🎯 Purpose / Motivation

clearCache() was exported but did nothing, which could mislead callers into thinking cached claimable values were being invalidated. On cache hits, the service returned { ...cachedEntry, cached: true, cachedAt } as any, bypassing TypeScript because cachedAt was not part of the declared result type.

🛠️ Changes Made

• Removed the empty clearCache() method (no callers in the codebase).
• Added cachedAt?: string to ClaimableAmountResult so cache-hit metadata is typed correctly.
• Removed the as any cast on the cache-hit return path.

🧪 How to Test

1. From backend/, run npm test -- claimable.service.test.ts.
2. Confirm all tests pass, especially the cache behavior test (uses cache for repeated request with same stream state + timestamp).
3. Optionally call getClaimableAmount twice with the same input and verify the second response has cached: true and an ISO cachedAt string when served from cache.

⚠️ Breaking Changes

• None. clearCache() had no references; cachedAt remains optional on responses.

🔗 Related Issues

Closes #547

✅ Checklist

• Code builds successfully
• Tests added/updated
• No console errors
• Documentation updated (if needed)