docs: release durable event contract

CHANGELOG.md

@@ -1,5 +1,27 @@
 # Changelog
 
+## v0.4.0 - 2026-06-17
+
+Added:
+
+- `durableevent` package with the shared wire contract for durable
+  cross-service events.
+- Event validation, payload normalization and stable idempotency key helper.
+
+Migration:
+
+1. Push `portal-common` commit and tag `v0.4.0`.
+2. In each producer/consumer service, update:
+
+   ```bash
+   go get gitea.estateliga.work/admin/portal-common@v0.4.0
+   go mod tidy
+   ```
+
+3. Use `durableevent.New(...)` for critical cross-service events and publish
+   the JSON envelope to the selected durable transport.
+4. Keep consumers idempotent by storing or checking `idempotency_key`.
+
 ## v0.3.0 - 2026-06-17
 
 Added: