feat(server): add syslog priority prefixes for systemd-journald (#5192)

* fix: add syslog priority prefixes for systemd-journald

When running under systemd, all log messages were assigned priority 3
(error) by journald because navidrome wrote plain text to stderr without
syslog priority prefixes.

Add a journalFormatter that wraps the existing logrus formatter and
prepends <N> syslog priority prefixes (RFC 5424) to each log line.
The formatter is automatically enabled when the JOURNAL_STREAM
environment variable is set (indicating the process is managed by
systemd).

Priority mapping:
- Fatal/Panic → <2>/<0> (crit/emerg)
- Error → <3> (err)
- Warn → <4> (warning)
- Info → <6> (info)
- Debug/Trace → <7> (debug)

Fixes #5142

* test: refactor journalFormatter tests to use Ginkgo and DescribeTable

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>
Co-authored-by: Deluan <deluan@navidrome.org>

conf/configuration.go

@@ -340,6 +340,10 @@ func Load(noConfigDump bool) {
 			os.Exit(1)
 		}
 		log.SetOutput(out)
+	} else if os.Getenv("JOURNAL_STREAM") != "" {
+		// When running under systemd, prepend syslog priority prefixes so
+		// journald assigns the correct severity to each log line.
+		log.EnableJournalFormat()
 	}
 
 	log.SetLevelString(Server.LogLevel)