fix(deploy): make the lobby image actually build

The lobby image had never been built and two defects blocked it: the felis image .dockerignore excluded plugins/* and only re-included limbo/shared, so the lobby Dockerfile's COPY plugins/paper landed empty; and the plugin stage used eclipse-temurin:21-jdk, which ships no gradle (and the tree vendors no wrapper), failing with 'gradle: not found'. Re-include plugins/paper and build the paper plugin on gradle:8.14-jdk21, matching the limbo image. Verified: both images build and boot (limbo /healthz 200 on 25565; lobby reaches 'Done' with felis-paper enabled).

deploy/lobby/Dockerfile

@@ -17,7 +17,9 @@
 # token.
 
 # ---- build the felis-paper plugin jar (Paper API is Java 21) ----
-FROM eclipse-temurin:21-jdk AS plugin
+# gradle:8.14-jdk21 — an official Gradle image on JDK 21 (this tree vendors no Gradle
+# wrapper, and a bare JDK image ships no `gradle`). JDK 21 matches the Paper API.
+FROM gradle:8.14-jdk21 AS plugin
 WORKDIR /src
 COPY plugins/paper/ ./plugins/paper/
 COPY plugins/shared/ ./plugins/shared/