fix(setup): reduce friction in installation procedure

- Add Language default ("jp") so missing field no longer produces empty
  string, and include language selector in setup wizard
- Add --setup flag to re-run wizard even when config.json exists,
  providing a recovery path for corrupted configs
- Auto-apply seed data on fresh databases so users who skip the wizard
  still get shops, events, and gacha
- Fix stale docs referencing non-existent init/setup.sh and
  schemas/patch-schema/ in docker/README, CONTRIBUTING, and README

config/config.go

@@ -342,6 +342,7 @@ func getOutboundIP4() (net.IP, error) {
 // config.json (just database credentials) produces a fully working server.
 func registerDefaults() {
 	// Top-level settings
+	viper.SetDefault("Language", "jp")
 	viper.SetDefault("BinPath", "bin")
 	viper.SetDefault("HideLoginNotice", true)
 	viper.SetDefault("LoginNotices", []string{