Fixes manifest hard dels (#92283)

## About The Pull Request

<img width="378" height="199" alt="1eSxYbsh0e"
src="https://github.com/user-attachments/assets/e8a658ca-c1c4-48fe-bb51-c77c85a7f824"
/>

Noticed some hard dels here, does some light refactoring/code
improvement to ensure that doesn't happen.

Crates shouldn't really be owning a hard ref to the manifest in the
first place since they are detachable. Removes some code duplication in
favor of calling `tear_manifest()` which has the safety check to prevent
`forceMove()`ing a qdeleted manifest out of nullspace.

## Why It's Good For The Game

Less server hiccups.

## Changelog

Nothing players will notice besides less server hiccups.

code/modules/bitrunning/server/loot.dm

@@ -46,7 +46,7 @@
 	certificate.update_appearance()
 
 	var/obj/structure/closet/crate/secure/bitrunning/decrypted/reward_cache = new(src, generated_domain, bonus)
-	reward_cache.manifest = certificate
+	reward_cache.manifest = WEAKREF(certificate)
 	reward_cache.update_appearance()
 
 	if(can_generate_tech_disk(grade))