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

settings.gradle

@@ -3,5 +3,6 @@ rootProject.name = 'discord-bot'
 dependencyResolutionManagement {
     repositories {
         mavenCentral()
+        maven { url = "https://maven.lavalink.dev/releases" }
     }
 }