feat: add Prometheus metrics endpoint and Docker image (PLAN.md 4.2)

Add internal/metrics package with dedicated Prometheus registry exposing
SSH connection, auth attempt, session, and build info metrics. Wire into
SSH server (4 instrumentation points) and web server (/metrics endpoint).
Add dockerImage output to flake.nix via dockerTools.buildLayeredImage.
Bump version to 0.7.0.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/server/server_test.go

@@ -12,6 +12,7 @@ import (
 	"time"
 
 	"git.t-juice.club/torjus/oubliette/internal/config"
+	"git.t-juice.club/torjus/oubliette/internal/metrics"
 	"git.t-juice.club/torjus/oubliette/internal/storage"
 	"golang.org/x/crypto/ssh"
 )
@@ -120,7 +121,7 @@ func TestIntegrationSSHConnect(t *testing.T) {
 
 	logger := slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelDebug}))
 	store := storage.NewMemoryStore()
-	srv, err := New(cfg, store, logger)
+	srv, err := New(cfg, store, logger, metrics.New("test"))
 	if err != nil {
 		t.Fatalf("creating server: %v", err)
 	}