fix(logger): render full logfmt line as journal MESSAGE

The native journal handler only put the bare msg string in MESSAGE,
so default `journalctl` output lost all slog attributes (env, name,
ip_version, etc.) that were visible with the old TextHandler path.

Wrap the slog-journal handler so each record's MESSAGE is rendered
through slog.TextHandler — producing the same `level=INFO msg="..."
key=val` format as before — while still emitting every attribute as
a structured journal field for `journalctl -o verbose` / field-based
filtering.

Also fix go.mod: slog-journal is a direct dependency.

logger/journald.go

@@ -1,7 +1,10 @@
 package logger
 
 import (
+	"bytes"
+	"context"
 	"log/slog"
+	"slices"
 
 	slogjournal "github.com/systemd/slog-journal"
 )
@@ -22,7 +25,71 @@ func newJournalHandler(opts *slog.HandlerOptions) slog.Handler {
 	if err != nil {
 		return nil
 	}
-	return h
+	return &journalMessageHandler{inner: h, level: opts.Level}
+}
+
+// journalMessageHandler wraps a slog-journal handler so each record's
+// MESSAGE field contains the full logfmt line (level, msg, and all
+// attributes) — matching the pre-journal stderr TextHandler output so
+// plain `journalctl` remains readable. Attributes are still delivered as
+// structured journal fields, so `journalctl -o verbose` or
+// `--output-fields=` keep working for filtering.
+type journalMessageHandler struct {
+	inner slog.Handler
+	level slog.Leveler
+	ops   []handlerOp
+}
+
+type handlerOp struct {
+	attrs []slog.Attr // non-nil for WithAttrs
+	group string      // non-empty for WithGroup
+}
+
+func (h *journalMessageHandler) Enabled(ctx context.Context, level slog.Level) bool {
+	return h.inner.Enabled(ctx, level)
+}
+
+func (h *journalMessageHandler) Handle(ctx context.Context, r slog.Record) error {
+	var buf bytes.Buffer
+	var th slog.Handler = slog.NewTextHandler(&buf, &slog.HandlerOptions{
+		Level:       h.level,
+		ReplaceAttr: logRemoveTime,
+	})
+	for _, op := range h.ops {
+		if op.group != "" {
+			th = th.WithGroup(op.group)
+		} else {
+			th = th.WithAttrs(op.attrs)
+		}
+	}
+	if err := th.Handle(ctx, r); err != nil {
+		return err
+	}
+	msg := bytes.TrimRight(buf.Bytes(), "\n")
+
+	nr := r.Clone()
+	nr.Message = string(msg)
+	return h.inner.Handle(ctx, nr)
+}
+
+func (h *journalMessageHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
+	if len(attrs) == 0 {
+		return h
+	}
+	h2 := *h
+	h2.ops = append(slices.Clip(h.ops), handlerOp{attrs: attrs})
+	h2.inner = h.inner.WithAttrs(attrs)
+	return &h2
+}
+
+func (h *journalMessageHandler) WithGroup(name string) slog.Handler {
+	if name == "" {
+		return h
+	}
+	h2 := *h
+	h2.ops = append(slices.Clip(h.ops), handlerOp{group: name})
+	h2.inner = h.inner.WithGroup(name)
+	return &h2
 }
 
 // journalReplaceAttr sanitizes slog attribute keys so they satisfy the