Add SMTP authentication using Cloudron sendmail credentials

Configure Museum to use CLOUDRON_MAIL_SMTP_USERNAME and
CLOUDRON_MAIL_SMTP_PASSWORD for authenticated SMTP relay.
This fixes the "550 I cannot deliver mail" error by properly
authenticating with the Cloudron sendmail addon.

CloudronManifest.json

@@ -7,7 +7,7 @@
   "contactEmail": "contact@ente.io",
   "website": "https://ente.io",
   "tagline": "Open source, end-to-end encrypted photo backup",
-  "version": "0.3.6",
+  "version": "0.3.7",
   "upstreamVersion": "git-main",
   "healthCheckPath": "/health",
   "httpPort": 3080,

start.sh

@@ -122,11 +122,11 @@ HASH_KEY_FILE="$SECRETS_DIR/hash_key"
 JWT_SECRET_FILE="$SECRETS_DIR/jwt_secret"
 SESSION_SECRET_FILE="$SECRETS_DIR/session_secret"
 
-SMTP_HOST="mail"
-SMTP_PORT="2525"
+SMTP_HOST="${CLOUDRON_MAIL_SMTP_SERVER:-mail}"
+SMTP_PORT="${CLOUDRON_MAIL_SMTP_PORT:-2525}"
 SMTP_ENCRYPTION=""
-SMTP_USERNAME=""
-SMTP_PASSWORD=""
+SMTP_USERNAME="${CLOUDRON_MAIL_SMTP_USERNAME:-}"
+SMTP_PASSWORD="${CLOUDRON_MAIL_SMTP_PASSWORD:-}"
 SMTP_EMAIL="${CLOUDRON_MAIL_FROM:-no-reply@$RP_ID}"
 SMTP_SENDER_NAME="${CLOUDRON_MAIL_FROM_DISPLAY_NAME:-Ente}"