Adds minimum pop restrictions to spy (#5740)

## About The Pull Request

Title. Restricts roundstart and midround spy spawns to a minimum
population of 15. Tested and works as expected via checking storyteller
panel.
## Why It's Good For The Game

Spies are effectively traitors, except that they snowball much harder,
given they can theoretically steal an infinite number of items, and thus
gain an infinite amount of traitor gear (and in some cases, even more
powerful gear)
On low-pop, however, spies become significantly more oppressive. With
less people around, breaking into restricted areas and stealing
equipment becomes far, far easier, which makes it easier to get
equipment to further encourage yet more stealing, etc. It snowballs much
harder since there are less chances for them to be caught and therefore
targeted. This is especially so if a spy happens to volunteer for acting
captaincy with no command crew available - at that point they just have
free reign over the entire station, and access to a hell of a lot of
powerful equipment as a result - enough that even if that low population
WANTED to stop that spy, they may well not even be able to.

Initially I settled on 20 as a minimum requirement, but upon seeing 20
as the restriction for malf AI, I settled on 15 instead - just a touch
lower. They're still overall a fairly quiet antag type compared to
others, so 15 felt right to me - though this should be discussed.
## Proof Of Testing
Clipped from storyteller panel
<img width="874" height="104" alt="2026-06-04_20-56-45"
src="https://github.com/user-attachments/assets/68d03932-fc1a-40da-9d22-73f4fbcb66a1"
/>

## Changelog
:cl:Fogspot
balance: Added minimum population restrictions to spy spawns.
/🆑

modular_zubbers/code/modules/storyteller/event_defines/crewset/spies.dm

@@ -6,12 +6,16 @@
 	antag_datum = /datum/antagonist/spy
 	weight = 8
 	maximum_antags_global = 4
+	
+	min_players = 15
 
 	tags = list(TAG_CREW_ANTAG)
 
 /datum/round_event_control/antagonist/solo/spy/midround
 	name = "Spies (Midround)"
 	roundstart = FALSE
+	
+	min_players = 15
 
 /datum/round_event_control/antagonist/solo/spy/event
 	name = "Event Generated Spy"