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/internal/config/config_test.go

@@ -0,0 +1,46 @@
+package config
+
+import "testing"
+
+// TestLoadDefaults verifies that Load applies defaults when the environment is
+// empty.
+func TestLoadDefaults(t *testing.T) {
+	t.Setenv("BACKEND_HTTP_ADDR", "")
+	t.Setenv("BACKEND_LOG_LEVEL", "")
+
+	c, err := Load()
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if c.HTTPAddr != defaultHTTPAddr {
+		t.Errorf("HTTPAddr = %q, want %q", c.HTTPAddr, defaultHTTPAddr)
+	}
+	if c.LogLevel != defaultLogLevel {
+		t.Errorf("LogLevel = %q, want %q", c.LogLevel, defaultLogLevel)
+	}
+}
+
+// TestLoadOverrides verifies that environment variables override the defaults.
+func TestLoadOverrides(t *testing.T) {
+	t.Setenv("BACKEND_HTTP_ADDR", "127.0.0.1:9090")
+	t.Setenv("BACKEND_LOG_LEVEL", "debug")
+
+	c, err := Load()
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if c.HTTPAddr != "127.0.0.1:9090" {
+		t.Errorf("HTTPAddr = %q, want %q", c.HTTPAddr, "127.0.0.1:9090")
+	}
+	if c.LogLevel != "debug" {
+		t.Errorf("LogLevel = %q, want %q", c.LogLevel, "debug")
+	}
+}
+
+// TestLoadRejectsInvalidLevel verifies that an unknown log level is rejected.
+func TestLoadRejectsInvalidLevel(t *testing.T) {
+	t.Setenv("BACKEND_LOG_LEVEL", "verbose")
+	if _, err := Load(); err == nil {
+		t.Fatal("Load: expected an error for an invalid log level, got nil")
+	}
+}