Stage 0: scaffold monorepo, backend skeleton, docs, CI

- go.work (Go 1.26.3) with backend module; deps added incrementally (gin+zap only)

- backend: /healthz + /readyz, env config, graceful shutdown

- docs: ARCHITECTURE, FUNCTIONAL (+ru mirror), TESTING

- PLAN.md (stage tracker + per-stage open details) and CLAUDE.md (per-stage workflow)

- .gitea go-unit CI (gofmt/vet/build/test)

backend/cmd/backend/main.go

@@ -0,0 +1,48 @@
+// Command backend is the Scrabble platform's internal domain service. At this
+// stage it boots the HTTP listener with the infrastructure probes only; the
+// domain modules described in PLAN.md are added by later stages.
+package main
+
+import (
+	"context"
+	"log"
+	"os/signal"
+	"syscall"
+
+	"go.uber.org/zap"
+
+	"scrabble/backend/internal/config"
+	"scrabble/backend/internal/server"
+)
+
+func main() {
+	cfg, err := config.Load()
+	if err != nil {
+		log.Fatalf("backend: load config: %v", err)
+	}
+
+	logger, err := newLogger(cfg.LogLevel)
+	if err != nil {
+		log.Fatalf("backend: build logger: %v", err)
+	}
+	defer func() { _ = logger.Sync() }()
+
+	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
+	defer stop()
+
+	srv := server.New(cfg.HTTPAddr, logger)
+	if err := srv.Run(ctx); err != nil {
+		logger.Fatal("backend: server terminated", zap.Error(err))
+	}
+}
+
+// newLogger builds a production JSON logger at the given level.
+func newLogger(level string) (*zap.Logger, error) {
+	var lvl zap.AtomicLevel
+	if err := lvl.UnmarshalText([]byte(level)); err != nil {
+		return nil, err
+	}
+	cfg := zap.NewProductionConfig()
+	cfg.Level = lvl
+	return cfg.Build()
+}