feat(logger): deliver stderr to systemd journal with per-record priority

When stderr is connected to journald, switch from the plain text handler
to the native journal protocol via github.com/systemd/slog-journal. Each
record now carries PRIORITY, MESSAGE, and slog attributes as structured
fields, so journalctl -p and LogLevelMax= in unit files filter by
severity instead of dropping everything at once.

Detection follows systemd.exec(5): parse JOURNAL_STREAM as <dev>:<ino>
and fstat(2) stderr to confirm the match, guarding against processes
that inherit the env var but have stderr redirected elsewhere. The
fstat path is Linux-only; other platforms fall through to the existing
text handler.

Also honour DEBUG_INVOCATION by raising stderr to Debug level, matching
the behaviour of systemd.service(5) RestartMode=debug.

logger/logger.go

@@ -128,6 +128,15 @@ func setupStdErrHandler() slog.Handler {
 		}
 	}
 
+	// DEBUG_INVOCATION is set by systemd on a restart attempt of a unit
+	// configured with RestartMode=debug when the previous start failed.
+	// Treat it as a request to raise stderr verbosity to Debug so the
+	// next failing cycle yields maximum diagnostics. OTLPLevel is left
+	// alone — that stays under server/admin control.
+	if os.Getenv("DEBUG_INVOCATION") != "" {
+		Level.Set(slog.LevelDebug)
+	}
+
 	logOptions := &slog.HandlerOptions{Level: Level}
 
 	if len(os.Getenv("INVOCATION_ID")) > 0 {
@@ -137,8 +146,15 @@ func setupStdErrHandler() slog.Handler {
 		logOptions.ReplaceAttr = logRemoveTime
 	}
 
+	var base slog.Handler
+	if h := newJournalHandler(logOptions); h != nil {
+		base = h
+	} else {
+		base = slog.NewTextHandler(os.Stderr, logOptions)
+	}
+
 	logHandler := slogtraceid.OtelHandler{
-		Next: slog.NewTextHandler(os.Stderr, logOptions),
+		Next: base,
 	}
 
 	return logHandler