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>

lib/notifications/email.ts

@@ -1,5 +1,5 @@
 import 'server-only'
-import { Resend } from 'resend'
+import { sendEmail } from '@/lib/notifications/mailer'
 import { asAdmin } from '@/lib/db/with-user'
 import { users, incidents, sites } from '@/lib/db/schema'
 import { and, eq, inArray } from 'drizzle-orm'
@@ -55,9 +55,5 @@ export async function sendNewIncidentEmail(
     incident_id: incidentId,
   })
 
-  const resend = new Resend(process.env.RESEND_API_KEY)
-  const from = process.env.RESEND_FROM_EMAIL ?? 'onboarding@resend.dev'
-
-  const { error } = await resend.emails.send({ from, to, subject, html, text })
-  if (error) console.error('Resend error:', error)
+  await sendEmail({ to, subject, html, text })
 }