feat(streams): implement stream top-up endpoint (#595)

[fytgian]

Summary

Closes Vera3289#595

Adds the ability for employers to top up existing streams with additional funds without cancellation.

Changes

Modified files

• api/routes/streams.js — Added POST /:stream_id/top-up route

How it works

1. Validates stream_id (integer ≥ 1), employer (Stellar G-address), and amount (positive integer string)
2. Calls get_stream on the Soroban contract to verify the stream exists
3. Rejects with 400 if the stream is not in Active status (cannot top-up Paused/Cancelled/Exhausted streams)
4. Submits top_up(employer, stream_id, amount) transaction to the Soroban contract
5. Invalidates the stream cache so subsequent GET /:stream_id reflects the updated deposit
6. Returns { success, stream_id, amount_added, transaction_hash }

Acceptance Criteria

Criterion	Status
Top-up endpoint implementation	✅ POST /v1/api/streams/:stream_id/top-up
Validation of existing stream	✅ get_stream call + status check before submission
Fund transfer and escrow update	✅ top_up Soroban contract call
Event logging	✅ Soroban contract emits on-chain events; cache invalidated
Fee handling	✅ Handled by Soroban contract; BASE_FEE set in stellarService
Transaction confirmation	✅ Returns transaction_hash from confirmed tx
Input validation	✅ express-validator on all fields