fix: re-enable CI lint job and fix ~65 lint errors (partial)

Re-enable the golangci-lint job in CI (disabled Oct 2025), update to
Go 1.25 and golangci-lint-action v7. Fix errcheck, gosimple S1009,
staticcheck SA4031 and SA2001 errors across 54 files. Remaining ~39
lint errors will be addressed in follow-up commits.

server/discordbot/discord_bot_test.go

@@ -356,15 +356,11 @@ func TestCommands_PasswordCommand(t *testing.T) {
 
 func TestDiscordBotStruct(t *testing.T) {
 	// Test that the DiscordBot struct can be initialized
-	bot := &DiscordBot{
+	_ = &DiscordBot{
 		Session:      nil, // Can't create real session in tests
 		MainGuild:    nil,
 		RelayChannel: nil,
 	}
-
-	if bot == nil {
-		t.Error("Failed to create DiscordBot struct")
-	}
 }
 
 func TestOptionsStruct(t *testing.T) {