feat: add toISOTimestamp utility and apply consistently across routes

[sadiqabubakar826-del]

Add toISOTimestamp utility and apply consistently across all routes

Adds a toISOTimestamp(value) utility function to src/utils/response.js that normalizes all
timestamp fields in API responses to ISO 8601 format (YYYY-MM-DDTHH:mm:ss.sssZ).

What changed

• src/utils/response.js — added toISOTimestamp(value) handling Unix seconds, Unix milliseconds,
  Stellar date strings, and JS Date objects. formatTimestamp kept as a deprecated alias.
• src/routes/transactions.js — applied to createdAt on transaction records, operation records,
  and batch-status results.
• src/routes/account.js — applied to firstSeen, lastSeen (analytics), lastTransactionAt
  (inactivity), createdAt (payments), and timestamp (offer history).
• src/routes/networkStatus.js — applied to closedAt on the latest ledger.
• src/utils/formatTransaction.js — applied to created_at in the SSE stream formatter.
• tests/utils.toISOTimestamp.test.js — 10 tests covering null/undefined/empty, ISO strings,
  Date objects, Unix seconds, Unix milliseconds, and output format validation.
• tests/transactions.batch.test.js — updated assertions to match normalized .000Z suffix.

Why

Previously, timestamps were passed through as raw Horizon strings (e.g. 2024-07-01T12:00:00Z)
without normalization. The new utility ensures every timestamp field across all endpoints uses
a consistent, fully-normalized ISO 8601 format regardless of input type.

▸ Credits: 0.19 • Time: 10s
─────────────────────────────────────────────────────────────────────────────────────────────────
check out to new branch commit and push

The branch was already created, committed, and pushed in the previous step. There's nothing new
to commit — the working tree is clean:
● Shell cd /workspaces/stellarkit-api && git status && git log --oneline -3
On branch feat/iso-timestamp-utility
Your branch is up to date with 'origin/feat/iso-timestamp-utility'.
nothing to commit, working tree clean
e108082 feat: add toISOTimestamp utility and apply consistently across routes
...+3 lines (ctrl+o to toggle)

You're already on feat/iso-timestamp-utility, the commit is there, and it's up to date with the
remote. Nothing left to do — you can open a PR directly from:

https://github.com/sadiqabubakar826-del/stellarkit-api/pull/new/feat/iso-timestamp-utility

closes #213

[drips-wave]

@sadiqabubakar826-del Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits