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.

go.mod

@@ -16,6 +16,7 @@ require (
 	github.com/samber/slog-multi v1.8.0
 	github.com/segmentio/kafka-go v0.4.50
 	github.com/spf13/cobra v1.10.2
+	github.com/systemd/slog-journal v0.1.2
 	go.opentelemetry.io/contrib/bridges/otelslog v0.18.0
 	go.opentelemetry.io/contrib/exporters/autoexport v0.68.0
 	go.opentelemetry.io/contrib/instrumentation/github.com/labstack/echo/otelecho v0.68.0
@@ -71,7 +72,6 @@ require (
 	github.com/samber/slog-common v0.22.0 // indirect
 	github.com/spf13/afero v1.15.0 // indirect
 	github.com/spf13/pflag v1.0.10 // indirect
-	github.com/systemd/slog-journal v0.1.2 // indirect
 	github.com/valyala/bytebufferpool v1.0.0 // indirect
 	github.com/valyala/fasttemplate v1.2.2 // indirect
 	go.opentelemetry.io/auto/sdk v1.2.1 // indirect