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.go

@@ -0,0 +1,57 @@
+// Package config loads and validates the backend's runtime configuration from
+// the process environment.
+package config
+
+import (
+	"fmt"
+	"os"
+)
+
+// Config holds the backend's runtime configuration.
+type Config struct {
+	// HTTPAddr is the listen address of the HTTP listener (host:port).
+	HTTPAddr string
+	// LogLevel is the zap log level: "debug", "info", "warn" or "error".
+	LogLevel string
+}
+
+// Defaults applied when the corresponding environment variable is unset.
+const (
+	defaultHTTPAddr = ":8080"
+	defaultLogLevel = "info"
+)
+
+// Load reads the configuration from the environment, applies defaults for
+// unset variables, and validates the result.
+func Load() (Config, error) {
+	c := Config{
+		HTTPAddr: envOr("BACKEND_HTTP_ADDR", defaultHTTPAddr),
+		LogLevel: envOr("BACKEND_LOG_LEVEL", defaultLogLevel),
+	}
+	if err := c.validate(); err != nil {
+		return Config{}, err
+	}
+	return c, nil
+}
+
+// validate reports whether the configuration values are acceptable.
+func (c Config) validate() error {
+	switch c.LogLevel {
+	case "debug", "info", "warn", "error":
+	default:
+		return fmt.Errorf("config: invalid BACKEND_LOG_LEVEL %q", c.LogLevel)
+	}
+	if c.HTTPAddr == "" {
+		return fmt.Errorf("config: BACKEND_HTTP_ADDR must not be empty")
+	}
+	return nil
+}
+
+// envOr returns the value of the environment variable named key, or fallback
+// when the variable is unset or empty.
+func envOr(key, fallback string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return fallback
+}