Background tasks: advanced settings docs

[mattsodomsky]

Documentation for the background lane and per-task concurrency caps (lightward/mechanic-api#2290, lightward/mechanic-ui#2051).

New page at core/tasks/advanced-settings/background-tasks.md — the URL both UI "Learn more" links point to. Covers:

• The yield contract in merchant terms ("background runs wait until your other tasks are caught up"), including that busy shops may pause background work entirely and that this is the setting working, not a stuck task.
• Guaranteed progress (a run waiting for hours is periodically given a slot — deliberately phrased without pinning the exact threshold or cooldown).
• Manual retries re-enter the queue with the task's current settings (an earlier draft claimed a bypass that matched no code).
• Immediate reclassification of already-queued runs, both directions.
• An explicit statement that nothing is ever dropped — background runs queue without limit; the setting changes when they run, never whether.
• The bulk-operations expiry caveat as a warning hint (link path verified against the docs tree).
• Maximum concurrent runs, and how it differs from (and combines with) the background setting.

⚠️ Hold merge until mechanic-api#2290 deploys — GitBook publishes main immediately, and this describes behavior that doesn't exist yet.

🤖 Generated with Claude Code

[mattsodomsky]

Tracking issue with the full rollout checklist: lightward/mechanic-api#2293