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

Dockerfile

@@ -1,11 +1,11 @@
-FROM eclipse-temurin:21-jdk-alpine AS builder
+FROM eclipse-temurin:26-jdk-alpine AS builder
 WORKDIR /build
 COPY gradle gradle
 COPY gradlew settings.gradle build.gradle ./
 COPY src src
 RUN ./gradlew jar --stacktrace
 
-FROM eclipse-temurin:21-jre-alpine
+FROM eclipse-temurin:26-jre-alpine
 RUN addgroup -S bot && adduser -S bot -G bot
 WORKDIR /app
 COPY --from=builder /build/build/libs/discord-bot-1.0.0.jar app.jar
@@ -18,4 +18,4 @@ EXPOSE 8712
 HEALTHCHECK --interval=30s --timeout=5s --start-period=15s \
   CMD wget --no-verbose --tries=1 --spider http://localhost:8712/health || exit 1
 ENV DB_PATH=/app/data/bot.db
-ENTRYPOINT ["java", "-jar", "app.jar"]
+ENTRYPOINT ["java", "-Xmx512m", "-XX:+UseG1GC", "-XX:+UseStringDeduplication", "-jar", "app.jar"]