chore: Java 26 + Gradle 9.7.0, all 20 deps updated, 1GB mem limit

- build.gradle: Gradle toolchain languageVersion 26 (auto-detects /usr/lib/jvm,
  no JAVA_HOME export needed), .java-version file for IDEs
- gradle wrapper 9.3.1 -> 9.7.0, spotless 6.25->8.10.0, spotbugs 4.9.3->4.10.3
- Dockerfile: eclipse-temurin 26-jdk/jre-alpine + -Xmx512m G1GC StringDedup
- docker-compose: bot mem_limit 1g
- CI workflows: setup-java 26
- deps: jda 6.5.0, lavaplayer 2.2.7, logback 1.6.3, slf4j 2.0.18 (stable),
  caffeine 3.2.4, sqlite 3.53.2.1, hikaricp 7.1.0, okhttp 5.5.0,
  junit 5.13.4 + launcher 1.13.4, mockito 5.23.0, bytebuddy 1.18.12,
  checkstyle 13.10.0 (toolVersion), spotbugs plugin 6.4.8, jsoup 1.22.2
- 458 tests green, build green without JAVA_HOME export

build.gradle

@@ -4,7 +4,7 @@ plugins {
     id 'checkstyle'
     id 'pmd'
     alias(libs.plugins.spotbugs)
-    id 'com.diffplug.spotless' version '6.25.0'
+    id 'com.diffplug.spotless' version '8.10.0'
 }
 
 group = 'creator.loki'
@@ -31,12 +31,14 @@ dependencies {
     testImplementation libs.junit.jupiter
     testImplementation libs.mockito.core
     testImplementation libs.mockito.junit.jupiter
+    testImplementation libs.byte.buddy
     testRuntimeOnly libs.junit.platform.launcher
 }
 
 java {
-    sourceCompatibility = JavaVersion.VERSION_21
-    targetCompatibility = JavaVersion.VERSION_21
+    toolchain {
+        languageVersion = JavaLanguageVersion.of(26)
+    }
 }
 
 tasks.withType(JavaCompile) {
@@ -75,6 +77,7 @@ spotless {
 }
 
 checkstyle {
+    toolVersion = '13.10.0'
     configFile = file("${rootDir}/config/checkstyle/checkstyle.xml")
     ignoreFailures = true
 }
@@ -86,6 +89,7 @@ pmd {
 }
 
 spotbugs {
+    toolVersion = '4.10.3'
     excludeFilter = file("${rootDir}/config/spotbugs/exclude.xml")
     ignoreFailures = false
 }