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.go

@@ -0,0 +1,41 @@
+package log
+
+import (
+	"fmt"
+
+	"github.com/sirupsen/logrus"
+)
+
+// journalFormatter wraps a logrus.Formatter and prepends a syslog priority
+// prefix (<N>) to each log line. When stderr is captured by systemd-journald,
+// this prefix tells journald the correct severity for each message.
+//
+// See https://www.freedesktop.org/software/systemd/man/sd-daemon.html
+type journalFormatter struct {
+	inner logrus.Formatter
+}
+
+// levelToPriority maps logrus levels to syslog priority values.
+// The mapping follows RFC 5424 severity levels.
+var levelToPriority = map[logrus.Level]int{
+	logrus.PanicLevel: 0, // emerg
+	logrus.FatalLevel: 2, // crit
+	logrus.ErrorLevel: 3, // err
+	logrus.WarnLevel:  4, // warning
+	logrus.InfoLevel:  6, // info
+	logrus.DebugLevel: 7, // debug
+	logrus.TraceLevel: 7, // debug
+}
+
+func (f *journalFormatter) Format(entry *logrus.Entry) ([]byte, error) {
+	formatted, err := f.inner.Format(entry)
+	if err != nil {
+		return formatted, err
+	}
+	priority, ok := levelToPriority[entry.Level]
+	if !ok {
+		priority = 6 // default to info for unknown levels
+	}
+	prefix := []byte(fmt.Sprintf("<%d>", priority))
+	return append(prefix, formatted...), nil
+}