docs: add per-channel persistent download URL expiration env vars

Add PERSISTENT_DOWNLOAD_URL_EXPIRATION_SECONDS_EMAIL, _SLACK, and
_MSTEAMS for per-channel download link expiration overrides.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: charliedowler

self-host/customize-deployment/environment-variables.mdx

@@ -146,6 +146,9 @@ When enabled, CSV and dashboard ZIP exports return a stable Lightdash-hosted URL
 | :------------------------------------------- | :----------------------------------------------------------------------- |
 | `PERSISTENT_DOWNLOAD_URLS_ENABLED`           | Enables persistent download URLs (default=false)                         |
 | `PERSISTENT_DOWNLOAD_URL_EXPIRATION_SECONDS` | How long the persistent URL remains accessible (default=259200, 3 days). When persistent URLs are enabled, `S3_EXPIRATION_TIME` is ignored and each redirect generates a signed URL that expires after 5 minutes. |
+| `PERSISTENT_DOWNLOAD_URL_EXPIRATION_SECONDS_EMAIL` | Override expiration for email download links. Falls back to `PERSISTENT_DOWNLOAD_URL_EXPIRATION_SECONDS` when not set. |
+| `PERSISTENT_DOWNLOAD_URL_EXPIRATION_SECONDS_SLACK` | Override expiration for Slack download links. Falls back to `PERSISTENT_DOWNLOAD_URL_EXPIRATION_SECONDS` when not set. |
+| `PERSISTENT_DOWNLOAD_URL_EXPIRATION_SECONDS_MSTEAMS` | Override expiration for MS Teams download links. Falls back to `PERSISTENT_DOWNLOAD_URL_EXPIRATION_SECONDS` when not set. |
 
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