Add paused sandbox retention note to persistence docs (#134)

Clarify that paused sandboxes are kept indefinitely with no TTL
and can be resumed at any time.

Author: beran-t

docs/sandbox/persistence.mdx

@@ -294,6 +294,10 @@ If you resume the sandbox, the service will be accessible again but you need to
 - Pausing a sandbox takes approximately **4 seconds per 1 GiB of RAM**
 - Resuming a sandbox takes approximately **1 second**
 
+### Paused sandbox retention
+- Paused sandboxes are kept **indefinitely** — there is no automatic deletion or time-to-live limit
+- You can resume a paused sandbox at any time
+
 ### Continuous runtime limits
 - A sandbox can remain running (without being paused) for:
   - **24 hours** on the **Pro tier**