v2.3: Major refactoring, Lavalink music system, Spotify support, CI/CD

- Full refactoring: max 200 lines/file, max 3 files/dir, KISS/DRY/SOLID
- Lavalink client 3.4.0 + LavaSrc plugin (Spotify, Apple Music, Deezer)
- 3-bot architecture: main (!), music (q!), radio (r!)
- Filters: bassboost, nightcore, vaporwave, equalizer
- Slash commands for music (/play, /skip, etc.)
- Persistent queue in SQLite, auto-leave timer
- Radio manager with auto-reconnect
- Custom welcome GIF greetings via nekos.life
- JDA 5.6.1, gson 2.14, hikaricp 6.3, all deps updated
- Fixed YouTube playback via ytmsearch (LavaSrc)
- CI/CD: Docker publish + release workflows
- SpotBugs: all real bugs fixed, ignoreFailures=false
- Checkstyle: Google Style (adapted), 0 violations

build.gradle

@@ -3,20 +3,18 @@ plugins {
     id 'application'
     id 'checkstyle'
     alias(libs.plugins.spotbugs)
+    id 'com.diffplug.spotless' version '6.25.0'
 }
 
 group = 'creator.loki'
 version = '1.0.0'
 
-repositories {
-    mavenCentral()
-}
-
 dependencies {
     implementation(libs.jda) {
         exclude module: 'opus-java'
     }
     implementation libs.lavaplayer
+    implementation libs.lavalink.client
     implementation libs.dotenv
     implementation libs.slf4j.api
     implementation libs.logback.classic
@@ -62,13 +60,21 @@ jar {
     }
 }
 
+spotless {
+    java {
+        googleJavaFormat()
+        importOrder('java', 'javax', '')
+        removeUnusedImports()
+        trimTrailingWhitespace()
+        endWithNewline()
+    }
+}
+
 checkstyle {
     configFile = file("${rootDir}/config/checkstyle/checkstyle.xml")
-    maxErrors = 999
-    maxWarnings = 999
 }
 
 spotbugs {
     excludeFilter = file("${rootDir}/config/spotbugs/exclude.xml")
-    ignoreFailures = true
+    ignoreFailures = false
 }