Add backend with buffered zstd-compressed report storage

Introduce the Go backend (netwatch-server) with an HTTP API that
accepts telemetry reports and persists them as zstd-compressed JSONL
files. Reports are buffered in memory and flushed to disk when the
buffer reaches 10 MiB or every 60 seconds.

backend/internal/config/config.go

@@ -0,0 +1,86 @@
+// Package config loads application configuration from
+// environment variables, .env files, and config files.
+package config
+
+import (
+	"errors"
+	"log/slog"
+
+	"sneak.berlin/go/netwatch/internal/globals"
+	"sneak.berlin/go/netwatch/internal/logger"
+
+	_ "github.com/joho/godotenv/autoload" // loads .env file
+	"github.com/spf13/viper"
+	"go.uber.org/fx"
+)
+
+// Params defines the dependencies for Config.
+type Params struct {
+	fx.In
+
+	Globals *globals.Globals
+	Logger  *logger.Logger
+}
+
+// Config holds the resolved application configuration.
+type Config struct {
+	DataDir         string
+	Debug           bool
+	MetricsPassword string
+	MetricsUsername string
+	Port            int
+	SentryDSN       string
+	log             *slog.Logger
+	params          *Params
+}
+
+// New loads configuration from env, .env files, and config
+// files, returning a fully resolved Config.
+func New(
+	_ fx.Lifecycle,
+	params Params,
+) (*Config, error) {
+	log := params.Logger.Get()
+	name := params.Globals.Appname
+
+	viper.SetConfigName(name)
+	viper.SetConfigType("yaml")
+	viper.AddConfigPath("/etc/" + name)
+	viper.AddConfigPath("$HOME/.config/" + name)
+
+	viper.AutomaticEnv()
+
+	viper.SetDefault("DATA_DIR", "./data/reports")
+	viper.SetDefault("DEBUG", "false")
+	viper.SetDefault("PORT", "8080")
+	viper.SetDefault("SENTRY_DSN", "")
+	viper.SetDefault("METRICS_USERNAME", "")
+	viper.SetDefault("METRICS_PASSWORD", "")
+
+	err := viper.ReadInConfig()
+	if err != nil {
+		var notFound viper.ConfigFileNotFoundError
+		if !errors.As(err, &notFound) {
+			log.Error("config file malformed", "error", err)
+			panic(err)
+		}
+	}
+
+	s := &Config{
+		DataDir:         viper.GetString("DATA_DIR"),
+		Debug:           viper.GetBool("DEBUG"),
+		MetricsPassword: viper.GetString("METRICS_PASSWORD"),
+		MetricsUsername: viper.GetString("METRICS_USERNAME"),
+		Port:            viper.GetInt("PORT"),
+		SentryDSN:       viper.GetString("SENTRY_DSN"),
+		log:             log,
+		params:          &params,
+	}
+
+	if s.Debug {
+		params.Logger.EnableDebugLogging()
+		s.log = params.Logger.Get()
+	}
+
+	return s, nil
+}