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/log.go

@@ -145,6 +145,15 @@ func SetOutput(w io.Writer) {
 	defaultLogger.SetOutput(w)
 }
 
+// EnableJournalFormat wraps the current logger formatter with syslog
+// priority prefixes for systemd-journald. Only call this when output
+// goes to stderr and JOURNAL_STREAM is set.
+func EnableJournalFormat() {
+	loggerMu.Lock()
+	defer loggerMu.Unlock()
+	defaultLogger.Formatter = &journalFormatter{inner: defaultLogger.Formatter}
+}
+
 // Redact applies redaction to a single string
 func Redact(msg string) string {
 	r, _ := redacted.redact(msg)