feat(email): phase 6 — replace Resend with Brevo transactional email

Removes resend npm dependency. Adds lib/notifications/mailer.ts with a
raw-HTTP Brevo wrapper (sendEmail + sendPasswordResetEmail). All three
notification files updated to use the new wrapper.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -25,8 +25,7 @@
     "pdf-lib": "^1.17.1",
     "pg": "^8.22.0",
     "react": "^19.2.7",
-    "react-dom": "^19.2.7",
-    "resend": "^6.17.2"
+    "react-dom": "^19.2.7"
   },
   "devDependencies": {
     "@tailwindcss/postcss": "^4",