docs: Align docs on minimum sync frequency (30 seconds) (#5844)

## Summary

Aligns documentation on the minimum sync frequency. The actual minimum
is **30 seconds**, available on all plans.

### Changes

- **limits.mdx**: Added "Minimum sync frequency" section under "Function
execution limits" documenting the 30-second minimum
- **node.mdx** (SDK reference): Replaced "Min frequency: 5 minutes" with
a link to the limits page
- **spec.yaml** (API spec): Updated "Min frequency: 5 minutes" to "Min
frequency: 30 seconds"
- **implement-a-sync.mdx**: Replaced "15 seconds minimum interval" with
a link to the limits page
- **realtime-syncs.mdx**: Removed "high-frequency syncing add-on"
language, replaced with "available on all plans" and link to limits page

> **Note**: The `security.mdx` diff is a merge-base artifact from the
reused branch — the file content is identical to master.

Resolves NAN-5265

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Co-authored-by: mintlify[bot] <109931778+mintlify[bot]@users.noreply.github.com>
Co-authored-by: Robin Guldener <r.guldener@gmail.com>