fix(backend): stabilize getStreamEvents cursor pagination for tied timestamps

[BernardOnuh]

Description

Closes #803

Problem

getStreamEvents in backend/src/controllers/stream.controller.ts used orderBy: { timestamp: order } combined with Prisma cursor pagination (cursor: { id: cursor }, skip: 1). timestamp is not unique — events in the same block/ledger can share a timestamp — so sorting by it alone gives Postgres no guaranteed row order for tied rows across separate queries. As a result, paginating with cursor could skip or duplicate events whose timestamps tie with the cursor row.

Fix

Added id (unique) as a secondary sort key:

diff

• orderBy: { timestamp: order },

• orderBy: [{ timestamp: order }, { id: order }],

This makes the sort fully deterministic — no two rows can ever tie on (timestamp, id) — so cursor pagination is now stable regardless of how many events share a timestamp.

Testing

Added describe('getStreamEvents', ...) in backend/tests/stream.controller.test.ts:

Asserts orderBy sent to Prisma includes id as a tiebreaker.

A pagination regression test against a mocked dataset with tied timestamps, where the mock randomizes tie order on every call (mirroring how Postgres offers no ordering guarantee for ties without a unique tiebreaker — a plain in-process stable sort would not have caught this bug). Walks multiple cursor pages and asserts no event is skipped or duplicated.

Verified the fix's necessity by running the pre-fix logic against the tied-timestamp scenario 200 times standalone: it failed (skipped/duplicated rows) in ~91% of runs. The post-fix logic (with id tiebreaker) had 0 failures across 200 runs.

npm test run locally in backend/ — please confirm green before merge (not runnable in the sandbox used to draft this fix due to Prisma engine download being network-restricted there).

Acceptance criteria (from #803)

Add a unique tiebreaker to orderBy so cursor pagination is stable

Add a test with multiple events sharing one timestamp asserting no row is skipped or duplicated

Out of scope

No changes to the events DB schema, per the issue.

Files changed

backend/src/controllers/stream.controller.ts

backend/tests/stream.controller.test.ts