Use Cloudron STARTTLS port 2587 for SMTP

Switch from plain SMTP on port 2525 to STARTTLS on port 2587.
The Go smtp.SendMail function automatically handles STARTTLS
negotiation when encryption is empty, which is required by
Cloudron's sendmail addon on the STARTTLS port.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

CloudronManifest.json

@@ -7,7 +7,7 @@
   "contactEmail": "contact@ente.io",
   "website": "https://ente.io",
   "tagline": "Open source, end-to-end encrypted photo backup",
-  "version": "0.3.7",
+  "version": "0.3.8",
   "upstreamVersion": "git-main",
   "healthCheckPath": "/health",
   "httpPort": 3080,