added .env support for a systemd EnvironmentFile with an example and updated email sender

.env.example

@@ -0,0 +1,29 @@
+# Phoenix / App
+PHX_HOST=mixer.example.com
+PHX_SERVER=true
+PORT=4000
+SECRET_KEY_BASE=REPLACE_WITH_64_CHAR_SECRET  # generate with: mix phx.gen.secret
+
+# Database
+DATABASE_URL=ecto://USER:PASSWORD@HOST/DATABASE
+ECTO_IPV6=false
+POOL_SIZE=10
+
+# Clustering (leave blank if not using DNS-based clustering)
+DNS_CLUSTER_QUERY=
+
+# Auth
+TOKEN_SIGNING_SECRET=REPLACE_WITH_SECRET
+
+# S3 / Object Storage
+S3_ACCESS_KEY_ID=your-access-key-id
+S3_SECRET_ACCESS_KEY=your-secret-access-key
+S3_HOST=s3.amazonaws.com
+S3_BUCKET=your-bucket-name
+S3_ASSET_HOST=https://your-bucket.s3.amazonaws.com
+S3_SCHEME=https://
+S3_PORT=80
+S3_VIRTUAL_HOST=false
+
+# Email (Brevo)
+BREVO_API_KEY=your-brevo-api-key