fix(contracts,backend,frontend): test fee round-to-zero, BigInt timestamps, event write race, and stray label

[Psalmuel01]

Summary of Changes

This PR addresses issues #794, #830, #831, and #862 on branch feat/multi-issue-fix:

1. Contract Fee Rounding to Zero Test ([Testing] collect_fee fee-rounds-to-zero branch untested - positive fee rate but amount too small to charge #794)

   • Verified unit test test_no_fee_transfer_or_event_when_fee_rounds_to_zero in contracts/stream_contract/src/test.rs.
   • Confirms that when fee_rate_bps > 0 but amount * bps / 10_000 truncates to 0, no fee transfer or fee_collected event is emitted.

2. Backend BigInt Unix-Timestamp Schema & Migration ([Backend] Stream/StreamEvent Unix-timestamp columns typed Int overflow at 2038 (endTime can overflow today) #830)

   • Updated startTime, lastUpdateTime, endTime, and pausedAt on Stream and timestamp on StreamEvent from Int to BigInt in backend/prisma/schema.prisma.
   • Added migration 20260729170000_timestamp_bigint.
   • Updated soroban-event-worker.ts to compute timestamps with BigInt math so streams ending past Jan 2038 persist and round-trip correctly.

3. Controller vs Indexer Event Write Race Fix ([Backend] Action controllers write StreamEvent via create() + ledgerSequence:0, colliding with worker upsert on @@unique([transactionHash,eventType]) #831)

   • Updated action controller in backend/src/routes/v1/streams/withdraw.ts to write StreamEvent using upsert on transactionHash_eventType instead of create, eliminating P2002 500 errors when the worker indexes first.
   • Updated worker upsert calls in soroban-event-worker.ts so update: sets ledgerSequence and timestamp, correctly overwriting placeholder ledgerSequence: 0 entries.
   • Added regression test backend/tests/eventRace.test.ts.

4. Frontend Stray Label Removal ([Frontend] Stray Token: labeled statement left inside handleWithdraw on stream detail page #862)

   • Verified removal of stray Token: JS labeled statement inside handleWithdraw in frontend/src/app/streams/[id]/stream-details-content.tsx.
   • Confirmed withdraw state toggling and clean lint execution.

---

Issue Reference

• Closes [Testing] collect_fee fee-rounds-to-zero branch untested - positive fee rate but amount too small to charge #794
• Closes [Backend] Stream/StreamEvent Unix-timestamp columns typed Int overflow at 2038 (endTime can overflow today) #830
• Closes [Backend] Action controllers write StreamEvent via create() + ledgerSequence:0, colliding with worker upsert on @@unique([transactionHash,eventType]) #831
• Closes [Frontend] Stray Token: labeled statement left inside handleWithdraw on stream detail page #862

Verification Steps

• cargo test in contracts/stream_contract
• npm test in backend
• npm run lint in frontend