docs: add v1.9.x changelog entry

apps/docs/changelog.mdx

@@ -3,6 +3,32 @@ title: "Changelog"
 description: "Product updates and announcements from Unsend"
 ---
 
+<Update label="March 2026" description="v1.9.x">
+
+### Campaign Personalization with Contact Books
+
+Campaigns now support contact-book variable registry for personalization, so you can compose messages with richer recipient data.
+
+### Double Opt-In Customization
+
+Double opt-in now supports customizable confirmation flows and copy to better match your onboarding and compliance needs.
+
+### API and SDK Coverage
+
+Contact book support was expanded in the SDK, dashboard analytics is now available in both the public API and SDK, and a public endpoint to delete campaigns was added.
+
+### Webhook Filtering by Domain
+
+Webhooks now support multi-domain filtering, making it easier to route and monitor events per domain.
+
+### Other improvements
+
+- Added `REDIS_KEY_PREFIX` for Redis ACL namespace isolation.
+- Submit contact add form with `Cmd/Ctrl+Enter`.
+- Restored contact search behavior after pagination.
+
+</Update>
+
 <Update label="February 2025" description="v1.8.0">
 
 <Frame>