feat: add pull piece cleanup job for expired pieces

[silent-cipher]

#504 (comment)

[Copilot]

Pull request overview

Adds a global pg-boss scheduled job to periodically delete expired pull_pieces registrations created by pull checks, and exposes configuration/docs for controlling its cadence.

Changes:

• Introduces pull_piece_cleanup as a new global scheduled job/queue and wires it into pg-boss scheduling, workers, and metrics.
• Adds PULL_PIECE_CLEANUP_INTERVAL_SECONDS (validated via Joi) and threads it through backend config and scheduling.
• Adds repository/service support for deleting expired pull_pieces, plus documentation updates.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
docs/environment-variables.md	Documents new PULL_PIECE_CLEANUP_INTERVAL_SECONDS env var and adds it to the quick reference table.
docs/checks/pull-check.md	Adds the new cleanup interval env var to pull-check docs.
apps/backend/src/pull-check/pull-piece.repository.ts	Adds deleteExpired() DB deletion helper for expired registrations.
apps/backend/src/pull-check/pull-check.service.ts	Adds deleteExpiredPullPieces() entry point for the scheduled cleanup job.
apps/backend/src/pull-check/pull-check.service.spec.ts	Updates pull-piece config fixture to include the new interval setting.
apps/backend/src/jobs/repositories/job-schedule.repository.ts	Updates pg-boss job-state mapping to include the new cleanup queue/type.
apps/backend/src/jobs/jobs.service.ts	Creates queue/worker, schedules pull_piece_cleanup, enqueues it, and updates metrics handling.
apps/backend/src/jobs/jobs.service.spec.ts	Updates config fixture and asserts the new worker queue is registered/created.
apps/backend/src/jobs/job-queues.ts	Defines PULL_PIECE_CLEANUP_QUEUE constant.
apps/backend/src/database/entities/job-schedule-state.entity.ts	Extends JobType union with pull_piece_cleanup.
apps/backend/src/config/app.config.ts	Adds env var validation and config plumbing for pullPieceCleanupIntervalSeconds.

[SgtPooki]

LGTM