Implement the backfill completion callback and replace mock job output

[priscaenoch]

Problem

src/api/backfill.ts leaves the job-completion callback unimplemented (// TODO: Send completion callback, around line 242) and returns fabricated output: a mock file URL and metadata (around line 214) and hardcoded mockCounts (around line 263). A backfill/export job that reports a fake file and never notifies completion cannot be relied on.

What needs to be done

• Implement the completion callback so callers/subscribers are notified when a backfill job finishes (or fails), reusing the webhook dispatcher if appropriate.
• Replace the mock file URL/metadata with the real output location (for example the S3/exports path the service already uses).
• Replace mockCounts with real counts derived from the job.
• If any part cannot yet be real, gate it behind an explicit default-off flag rather than returning fabricated data.

Files

• src/api/backfill.ts
• coordinate with src/exports/ and src/webhooks/

Acceptance deliverables

• Completed backfill jobs emit a real completion notification.
• Job responses reference the real output and real counts, or are flag-gated.
• All CI checks pass; the change cannot be merged until CI is green.

Tests to pass

• Test: on job completion the callback/notification fires.
• Test: the response reports the real output location and counts (dependencies mocked).

[ifygreg01-best]

The issue description is crystal clear, Maintainer I would love to be given the opportunity to work on issue and deliver on time

[grantfox-oss]

🦊 GrantFox — @ifygreg01-best has been assigned to this issue as part of the Third Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #48)
2. Your PR will be reviewed by the octraban maintainers

Good luck! Track your progress on GrantFox.