feat(testing): add vitest suite + guerrilla mail email pool

- Vitest unit tests for POST /api/erase: validation, form-only
  providers, all email providers, CRLF sanitization
- Snapshot on PROVIDER_DATA to catch provider config drift
- Mock via require cache to avoid nodemailer/sendmail dependency
- sendErasureMail call assertions (args + call count)
- fetch-test-emails.mjs script to refresh disposable email pool
- mailer: add Reply-To header pointing to requester email
- mailer: fallback to direct transport when sendmail unavailable

api/vitest.config.js

@@ -0,0 +1,14 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    environment: 'node',
+    server: {
+      deps: {
+        // Force Vite to transform local source files (not just node_modules)
+        // so vi.mock() can intercept require() calls inside CJS route handlers
+        inline: [/routes\//, /services\//],
+      },
+    },
+  },
+});