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

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-java@v4
         with:
-          java-version: "21"
+          java-version: "26"
           distribution: "temurin"
       - uses: gradle/actions/setup-gradle@v4
       - run: ./gradlew build