chore: shorten Slack backfill interval default

[akshaan]

Summary

• change the Slack backfill workflow default interval from 10 minutes to 5 minutes
• align the Helm chart default with the workflow default
• update Slack ETL docs and schedule test expectation

Validation

• uv run --project services/api ruff check workflows/slack_backfill.py services/api/tests/test_slack_sync.py
• direct import assertion confirmed slack_backfill.SCHEDULE interval_seconds is 300 with env unset

Note

• Focused pytest selection is blocked locally by the API test Postgres fixture because this machine has initdb without the paired postgres binary.

[github-actions]

Cloudflare Workers docs preview

https://pr-399-centaur-docs.porto.workers.dev