chore: Centralize env parsing, validation, defaults, and deprecation notices (#3487)

* chore: Centralize env parsing, defaults, deprecation

* wip

* test

* test

* tsc

* docs, more validation

* fix: Allow empty REDIS_URL (defaults to localhost)

* test

* fix: SLACK_MESSAGE_ACTIONS not bool

* fix: Add SMTP port validation

server/utils/robots.ts

@@ -1,8 +1,10 @@
+import env from "@server/env";
+
 const DISALLOW_ROBOTS = `User-agent: *
 Disallow: /`;
 
 export const robotsResponse = () => {
-  if (process.env.DEPLOYMENT !== "hosted") {
+  if (env.DEPLOYMENT !== "hosted") {
     return DISALLOW_ROBOTS;
   }