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_cv.txt

@@ -0,0 +1,7 @@
+Traceback (most recent call last):
+  File "<string>", line 1, in <module>
+    import json; d=json.load(open('color_variants.json')); [print(repr(v.get('baseName')),'outfitType=',v.get('outfitType'),'colors=',len(v.get('colors',[]))) for v in d['variants']]
+                                                                                                                                                                        ~^^^^^^^^^^^^
+KeyError: 'variants'
+total items 530
+Counter({None: 530})