docs: add per-channel persistent download URL expiration env vars (#407)

* 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>

* docs: add download link expiration section and update default to 7 days

- Add configuring download link expiration section to scheduled
  deliveries guide with link to env vars docs
- Update default expiration from 3 days to 7 days (604800 seconds)
- Update info box to reflect new default

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

* docs: revert default expiration back to 3 days

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

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: charliedowler

guides/how-to-create-scheduled-deliveries.mdx

@@ -6,10 +6,12 @@ description: "Scheduled deliveries let you send a dashboard or saved chart, and
 If your Lightdash organization has [Slack](/references/workspace/adding-slack-integration) set up, you can create or select an existing saved chart or dashboard, add a scheduled delivery to it, and tell Lightdash when you’d like the send out your update. You can set up as many scheduled deliveries to a saved chart or dashboard as you like, and if you make any changes to the content, Lightdash will update the scheduled deliveries the next time they’re sent.
 
 <Info>
-  **For security reasons, the delivered files expire after 3 days.**
+  **For security reasons, the delivered files expire after 3 days by default.**
 
 This means old emails or Slack shares will have broken images and there is not an option to retrieve them. If you want to keep the sent files, it's best to download them from the scheduled delivery.
 
+Self-hosted instances can [configure download link expiration](#configuring-download-link-expiration) per channel.
+
 </Info>
 
 ## Enabling scheduled deliveries
@@ -228,3 +230,7 @@ For example:
 - I have a scheduled delivery set to `Weekly on Monday at 15:00`
 - I change the project default time zone to `(UTC -5:00) America/New York`
 - my scheduled delivery will now be set to `Weekly on Monday at 10:00`
+
+## Configuring download link expiration
+
+By default, download links in scheduled deliveries expire after 3 days. Self-hosted instances can adjust this globally or per channel (email, Slack, MS Teams) using [persistent download URL environment variables](/self-host/customize-deployment/environment-variables#persistent-download-urls).

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. |
 
 ## Cache