Upgrade to Gradle 9.5.0; bump deps; tidy build configuration

[pambrose]

Summary

Build infrastructure cleanup, driven by the version-catalog parsing the Makefile now relies on (PR #1).

Versions

• Gradle wrapper 9.4.1 → 9.5.0
• Kotlin 2.3.20 → 2.3.21
• Ktor 3.4.2 → 3.4.3
• ben-manes versions plugin 0.53.0 → 0.54.0
• Add gradle and jvm keys to gradle/libs.versions.toml so the Makefile and build.gradle.kts both read them as a single source of truth.

build.gradle.kts

• Move group and version to gradle.properties.
• Replace magic strings with named vals (mainClassName, fatJarName, jvmVersion, buildFatJarTask, cleanTask, stageTask, preReleaseSuffixes).
• Pull JVM toolchain version from libs.versions.jvm instead of hardcoding 21.
• Drop the project-level repositories { mavenCentral() } block; it now lives centrally in settings.gradle.kts.

settings.gradle.kts

• Add pluginManagement (gradlePluginPortal + mavenCentral).
• Centralize project repositories under dependencyResolutionManagement with RepositoriesMode.FAIL_ON_PROJECT_REPOS so subprojects can't silently introduce their own repos.

gradle.properties

• Restore group / version (now the canonical home).
• Enable org.gradle.configuration-cache=true.
• Set org.gradle.jvmargs for larger heap, metaspace, and stack so Kotlin compilation stops OOMing in CI/Docker builds.

.gitignore

• Add .DS_Store.

Test plan

• ./gradlew build -x test succeeds locally with the new Gradle 9.5.0 wrapper.
• make build and make jar succeed.
• make upgrade-wrapper is a no-op (wrapper already at the pinned version).
• Heroku stage task still produces the fat JAR.
• Docker image (make build-docker) still builds and runs.

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