chore: scaffold Go module, codegen tooling, and shared runtime config

Sets up go.mod/go.sum, buf (proto codegen) configuration, and the
env-driven Config shared by the message-plane server and queue-operator.

internal/config/config.go

@@ -0,0 +1,80 @@
+package config
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+)
+
+// Config is the env/flag-driven runtime configuration shared by the
+// message-plane server and the queue-operator (design.md §10).
+type Config struct {
+	KafkaBrokers []string
+
+	RedisAddr     string
+	RedisPassword string
+	RedisDB       int
+
+	AuthentikIssuerURL string
+	AuthentikAudience  string
+
+	GRPCListenAddr string
+	HTTPListenAddr string
+
+	MaxMessageBodyBytes int
+
+	DedupWindowSeconds int
+}
+
+// Load reads configuration from environment variables, applying the
+// defaults documented in design.md.
+func Load() (*Config, error) {
+	cfg := &Config{
+		KafkaBrokers:        splitCSV(getEnv("KMSVC_KAFKA_BROKERS", "localhost:9092")),
+		RedisAddr:           getEnv("KMSVC_REDIS_ADDR", "localhost:6379"),
+		RedisPassword:       os.Getenv("KMSVC_REDIS_PASSWORD"),
+		AuthentikIssuerURL:  os.Getenv("KMSVC_AUTHENTIK_ISSUER_URL"),
+		AuthentikAudience:   os.Getenv("KMSVC_AUTHENTIK_AUDIENCE"),
+		GRPCListenAddr:      getEnv("KMSVC_GRPC_LISTEN_ADDR", ":9090"),
+		HTTPListenAddr:      getEnv("KMSVC_HTTP_LISTEN_ADDR", ":8080"),
+		MaxMessageBodyBytes: 256 * 1024, // 256KB cap, design.md §6
+		DedupWindowSeconds:  300,        // 5 minutes, design.md §4
+	}
+
+	if v := os.Getenv("KMSVC_REDIS_DB"); v != "" {
+		n, err := strconv.Atoi(v)
+		if err != nil {
+			return nil, fmt.Errorf("invalid KMSVC_REDIS_DB: %w", err)
+		}
+		cfg.RedisDB = n
+	}
+
+	if cfg.AuthentikIssuerURL == "" {
+		return nil, fmt.Errorf("KMSVC_AUTHENTIK_ISSUER_URL is required")
+	}
+	if cfg.AuthentikAudience == "" {
+		return nil, fmt.Errorf("KMSVC_AUTHENTIK_AUDIENCE is required")
+	}
+
+	return cfg, nil
+}
+
+func getEnv(key, def string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return def
+}
+
+func splitCSV(s string) []string {
+	parts := strings.Split(s, ",")
+	out := make([]string, 0, len(parts))
+	for _, p := range parts {
+		p = strings.TrimSpace(p)
+		if p != "" {
+			out = append(out, p)
+		}
+	}
+	return out
+}