add mailpit to dev shell and process-compose

Mailpit listens on 1025 (smtp) and 8025 (web ui). Backend now
depends on it as a healthy dep so dev signups can flush their
confirmation emails to mailpit instead of /dev/null.

flake.nix

@@ -24,6 +24,7 @@
             typescript
             postgresql
             process-compose
+            mailpit
           ];
 
           shellHook = ''

process-compose.yaml

@@ -11,12 +11,24 @@ processes:
       initial_delay_seconds: 1
       period_seconds: 2
 
+  mailpit:
+    command: mailpit --smtp 127.0.0.1:1025 --listen 127.0.0.1:8025
+    readiness_probe:
+      http_get:
+        host: 127.0.0.1
+        port: 8025
+        path: /
+      initial_delay_seconds: 1
+      period_seconds: 2
+
   backend:
     command: php artisan serve --host=127.0.0.1 --port=8000
     working_dir: ./backend
     depends_on:
       postgres:
         condition: process_healthy
+      mailpit:
+        condition: process_healthy
     readiness_probe:
       http_get:
         host: 127.0.0.1