Change license from AGPL to MIT

- Replace restrictive AGPL with permissive MIT license
- Update package.json license field
- Update README with license reference

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

package.json

@@ -9,7 +9,7 @@
   },
   "keywords": ["mail", "smtp", "brevo", "nodemailer"],
   "author": "",
-  "license": "ISC",
+  "license": "MIT",
   "dependencies": {
     "better-sqlite3": "^11.0.0",
     "dotenv": "^16.4.0",