Improve environment variable handling and reading (#5389)

* wip: better env var reading

* move most env vars to env.rs

* migrate more env vars

* more migration

* more migrations

* More migration

* 🦀 dotenvy is gone (almost)

* 🦀 dotenvy is gone 🦀

* Fix mural source account env var handling

* Remove defaults from admin key vars

* dummy commit to update github pr

* fix ci

apps/labrinth/src/routes/v3/threads.rs

@@ -7,6 +7,7 @@ use crate::database::models::image_item;
 use crate::database::models::notification_item::NotificationBuilder;
 use crate::database::models::thread_item::ThreadMessageBuilder;
 use crate::database::redis::RedisPool;
+use crate::env::ENV;
 use crate::file_hosting::{FileHost, FileHostPublicity};
 use crate::models::ids::{ThreadId, ThreadMessageId};
 use crate::models::images::{Image, ImageContext};
@@ -631,9 +632,8 @@ pub async fn message_delete(
         let images =
             database::DBImage::get_many_contexted(context, &mut transaction)
                 .await?;
-        let cdn_url = dotenvy::var("CDN_URL")?;
         for image in images {
-            let name = image.url.split(&format!("{cdn_url}/")).nth(1);
+            let name = image.url.split(&format!("{}/", ENV.CDN_URL)).nth(1);
             if let Some(icon_path) = name {
                 file_host
                     .delete_file(