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>

log/journal_test.go

@@ -0,0 +1,41 @@
+package log
+
+import (
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+	"github.com/sirupsen/logrus"
+)
+
+var _ = Describe("journalFormatter", func() {
+	var formatter *journalFormatter
+
+	BeforeEach(func() {
+		inner := &logrus.TextFormatter{
+			DisableTimestamp: true,
+			DisableColors:    true,
+		}
+		formatter = &journalFormatter{inner: inner}
+	})
+
+	DescribeTable("prefixes log lines with syslog priority",
+		func(level logrus.Level, expectedPrefix string) {
+			entry := &logrus.Entry{
+				Logger:  logrus.New(),
+				Level:   level,
+				Message: "test message",
+				Data:    logrus.Fields{},
+			}
+			out, err := formatter.Format(entry)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(string(out)).To(HavePrefix(expectedPrefix))
+		},
+		Entry("error", logrus.ErrorLevel, "<3>"),
+		Entry("warning", logrus.WarnLevel, "<4>"),
+		Entry("info", logrus.InfoLevel, "<6>"),
+		Entry("debug", logrus.DebugLevel, "<7>"),
+		Entry("trace", logrus.TraceLevel, "<7>"),
+		Entry("fatal", logrus.FatalLevel, "<2>"),
+		Entry("panic", logrus.PanicLevel, "<0>"),
+		Entry("unknown level defaults to info", logrus.Level(99), "<6>"),
+	)
+})