feat: admin endpoints to reset username sync flags

Add POST /v1/admin/users/{pubkey}/reset-username and
POST /v1/admin/users/reset-usernames to clear manual_username
and last_synced_at so nostr profile sync re-evaluates users.
Includes OpenAPI docs, audit actions, and tests.

internal/audit/audit.go

@@ -14,6 +14,8 @@ const (
 
 	ActionUserAdded            = "user.added"
 	ActionUserUsernameChanged  = "user.username_changed"
+	ActionUserUsernameReset    = "user.username_reset"
+	ActionAllUsernamesReset    = "users.usernames_reset"
 	ActionUserExtended         = "user.extended"
 	ActionUserDeleted          = "user.deleted"
 	ActionPaymentConfirmed     = "payment.confirmed"