feat(i18n): consume central translations from dev.escalated:escalated-locale

[mpge]

Summary

Wire the helpdesk's MessageSource to the new central translations artifact dev.escalated:escalated-locale, with a host-app override layer.

• New MessageSourceConfig registers a ReloadableResourceBundleMessageSource chaining two basenames (first match wins):
  1. classpath:i18n/overrides/messages — sparse host-app overrides (src/main/resources/i18n/overrides/)
  2. classpath:META-INF/escalated/locale/messages — shipped by the central artifact
• EscalatedAutoConfiguration @Imports the new config so it is registered automatically.
• Adds implementation(\"dev.escalated:escalated-locale:0.1.0\") to build.gradle.kts.
• Seeds src/main/resources/i18n/overrides/ with .gitkeep + a README explaining the convention.
• README + CHANGELOG updated.

Translation content in this repo is intentionally untouched — once the central artifact resolves on the classpath it shadows the existing messages_{locale}.properties files at the root of src/main/resources/. A follow-up will delete those files in this repo (and every host plugin) once the central artifact is verified end-to-end.

Assumed packaging convention

The central artifact is assumed to ship its bundles at:

```
META-INF/escalated/locale/messages_{locale}.properties
```

This avoids collisions with the host app's own messages.properties, mirrors how Spring Boot ships its own bundles under META-INF, and matches the basename chain wired here. If the central artifact ends up using a different path, only the CENTRAL_BASENAME constant in MessageSourceConfig needs to change.

Blocked on dev.escalated:escalated-locale v0.1.0 Maven Central publish

Expected CI failure: ./gradlew build will fail to resolve dev.escalated:escalated-locale:0.1.0 because the artifact has not yet been published to Maven Central. Sonatype + GPG setup for the new repo is still in flight. This PR is intentionally DRAFT until the artifact is reachable from mavenCentral().

Once the publish lands, ./gradlew build should resolve cleanly with no further changes here.

Test plan

• ./gradlew build succeeds after dev.escalated:escalated-locale:0.1.0 is on Maven Central
• App boots; MessageSource resolves status.open to a non-code value (i.e. a real translation, not the key) for at least one locale shipped by the central artifact (pt_BR, it, ja, …)
• Drop a sparse src/main/resources/i18n/overrides/messages_en.properties with status.open=Active and confirm the override wins over the central bundle
• Confirm Locale.ROOT / unknown locale falls through to the central default bundle (no setFallbackToSystemLocale)
• Follow-up PR: delete src/main/resources/messages_*.properties once central artifact is verified