fix(line): Use non-expiring URLs for image and video messages (#13949)

Images and videos sent from Chatwoot to LINE inboxes fail to display on
the LINE mobile app — users see expired markers, broken thumbnails, or
missing images. This happens because LINE mobile lazy-loads images
rather than downloading them immediately, and the ActiveStorage signed
URLs expire after 5 minutes.

Closes
https://linear.app/chatwoot/issue/CW-6696/line-messaging-with-image-or-video-may-not-show-when-client-inactive

## How to reproduce

1. Create a LINE inbox and start a chat from the LINE mobile app
2. Close the LINE mobile app
3. Send an image from Chatwoot to that chat
4. Wait 7-8 minutes (past the 5-minute URL expiration)
5. Open the LINE mobile app — the image is broken/expired

## What changed

- **`originalContentUrl`**: switched from `download_url` (signed, 5-min
expiry) to `file_url` (permanent redirect-based URL)
- **`previewImageUrl`**: switched to `thumb_url` (250px resized
thumbnail meeting LINE's 1MB/240x240 recommendation), with fallback to
`file_url` for non-image attachments like video

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Sojan Jose <sojan@pepalo.com>

app/services/line/send_on_line_service.rb

@@ -44,10 +44,15 @@ class Line::SendOnLineService < Base::SendOnChannelService
       # Support only image and video for now, https://developers.line.biz/en/reference/messaging-api/#image-message
       next unless attachment.file_type == 'image' || attachment.file_type == 'video'
 
+      # Use file_url (permanent redirect-based URL) instead of download_url (signed URL that expires in 5 minutes).
+      # LINE mobile app lazy-loads images and may fetch them well after the message is sent.
+      original_url = attachment.file_url
+      preview_url = attachment.thumb_url.presence || original_url
+
       {
         type: attachment.file_type,
-        originalContentUrl: attachment.download_url,
-        previewImageUrl: attachment.download_url
+        originalContentUrl: original_url,
+        previewImageUrl: preview_url
       }
     end
   end