Add background job retry and monitoring API

[alceops]

Summary

• adds a durable background_jobs table/model for async work tracking
• adds /jobs APIs to enqueue/list/inspect jobs and /jobs/run to execute due jobs
• implements bounded retry with exponential backoff, status/error monitoring, and a send_reminder job handler
• adds Redis-unavailable fallbacks for auth/session/cache paths so tests and dev runs stay resilient when Redis is down
• documents the retry-monitoring flow

Tests

• PYTHONPATH=. ../../.venv/bin/pytest tests (24 passed, 54 warnings)

Closes #130