feat(imap): add IMAP_READONLY_MODE feature flag to safeguard shared mailboxes

When enabled, IMAP processing will fetch and process attachments but
will NOT modify the mailbox state (no starring, labeling, deleting,
or flag changes). This allows preprod instances to safely share a
Gmail inbox with production without interfering with production
email processing.

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/tasks/imap_tasks.py

@@ -218,20 +218,23 @@ def pull_inbox(mailbox_key, host, port, username, password, use_ssl, delete_afte
             # We call the function without assigning its return value since it is not used.
             fetch_attachments_and_enqueue(email_message)
 
-            if is_gmail_host:
-                mark_as_processed_with_star(mail, num)
-                mark_as_processed_with_label(mail, num, label="Ingested")
+            if settings.imap_readonly_mode:
+                logger.info("Readonly mode: skipping mailbox modifications for %s in %s", msg_id, mailbox_key)
+            else:
+                if is_gmail_host:
+                    mark_as_processed_with_star(mail, num)
+                    mark_as_processed_with_label(mail, num, label="Ingested")
+
+                if delete_after_process:
+                    logger.info("Deleting message %s from %s", num.decode(), mailbox_key)
+                    mail.store(num, "+FLAGS", "\\Deleted")
+                else:
+                    mail.store(num, "-FLAGS", "\\Seen")
 
             processed_emails[msg_id] = datetime.now(timezone.utc).strftime("%Y-%m-%dT%H:%M:%S")
             save_processed_emails(processed_emails)
 
-            if delete_after_process:
-                logger.info("Deleting message %s from %s", num.decode(), mailbox_key)
-                mail.store(num, "+FLAGS", "\\Deleted")
-            else:
-                mail.store(num, "-FLAGS", "\\Seen")
-
-        if delete_after_process:
+        if not settings.imap_readonly_mode and delete_after_process:
             mail.expunge()
 
         mail.close()