admin: change a player's username from the Players panel

Add POST players/{id}/username + a Username field in the player detail's
Profile tab. Changes only the unique @handle (not the display name) and
rejects a collision outright rather than auto-suffixing, so the admin
gets the exact handle they typed. Same 2-24 char letters/digits/_/-
rule as the account API; logged to the audit trail as old -> new.

DorkNet.Server/Data/CUsersAlexaDocuments_lk.txt

@@ -0,0 +1,2 @@
+total 530
+outfitType None = 530