Adds a new contraband pinup poster (#91963)

## About The Pull Request
Adds "Double Rainbow" pinup poster :-)


![image](https://github.com/user-attachments/assets/ad211c8c-5c5c-4d9c-8ee7-5f4811e25724)

## Why It's Good For The Game
It's colorful! It's fun! Well suited for a speakeasy type situation.
Maybe? The choice is yours.

We have a pinup pool with two posters in it right now. Personally I
think it should have a few more.

## Changelog
🆑
add: double rainbow poster
/🆑

---------

Co-authored-by: san7890 <the@san7890.com>

code/game/objects/effects/posters/contraband.dm

@@ -13,7 +13,11 @@
 	name = "random pinup poster"
 	icon_state = "rolled_poster"
 	/// List of posters which make you feel a certain type of way
-	var/static/list/pinup_posters = list(/obj/structure/sign/poster/contraband/lizard, /obj/structure/sign/poster/contraband/lusty_xenomorph)
+	var/static/list/pinup_posters = list(
+		/obj/structure/sign/poster/contraband/lizard,
+		/obj/structure/sign/poster/contraband/lusty_xenomorph,
+		/obj/structure/sign/poster/contraband/double_rainbow,
+	)
 
 /obj/item/poster/random_contraband/pinup/Initialize(mapload, obj/structure/sign/poster/new_poster_structure)
 	poster_type = pick(pinup_posters)
@@ -827,3 +831,10 @@ MAPPING_DIRECTIONAL_HELPERS(/obj/structure/sign/poster/contraband/jovial, 32)
 	icon_state = "rolled_poster"
 
 MAPPING_DIRECTIONAL_HELPERS(/obj/structure/sign/poster/contraband/bojack, 32)
+
+/obj/structure/sign/poster/contraband/double_rainbow
+	name = "Double Rainbow"
+	desc = "It's so bright and vivid! What does this mean?"
+	icon_state = "double_rainbow"
+
+MAPPING_DIRECTIONAL_HELPERS(/obj/structure/sign/poster/contraband/double_rainbow, 32)