Docs/arq background worker

[PeakPy]

This PR adds documentation to clarify how ARQ is used as the background worker in this project and what teams should consider for production use.

Changes:

• Add a "Background jobs & ARQ" section to README.md:
  • Explain that ARQ is the default async worker built on Redis.
  • Note that ARQ is currently in maintenance mode rather than active feature development.
  • Describe when ARQ is a good fit (lightweight, Redis-based, fully async).
  • Mention that teams with heavy background-processing needs may want to plan a migration or swap ARQ for another worker behind the same abstractions.
  • Link to the existing background tasks guide.

Rationale:

• Issue Dependency & Security Review: JWT, Database, and Background Worker Concerns #248 requested clearer guidance around the choice of ARQ and its long-term implications.
• Making this explicit in the README helps users decide whether to keep ARQ or adapt the worker layer for their own needs.

Tests:

• Docs-only change; no code behavior modified.