refactor: move Go source files to src/ directory

Organize project structure by relocating all Go files to a src/ directory,
following standard Go project conventions and improving code organization.

src/config.go

@@ -0,0 +1,41 @@
+package main
+
+import (
+	"os"
+	"time"
+)
+
+type Config struct {
+	Port            string
+	APIToken        string
+	DataDir         string
+	CleanupInterval time.Duration
+	DBPath          string
+}
+
+func loadConfig() Config {
+	return Config{
+		Port:            getEnv("AUTH_PROXY_PORT", "8080"),
+		APIToken:        getEnv("AUTH_PROXY_API_TOKEN", ""),
+		DataDir:         getEnv("DATA_DIR", "/data"),
+		CleanupInterval: parseDuration("CLEANUP_INTERVAL", 60*time.Second),
+		DBPath:          getEnv("AUTH_PROXY_DB_PATH", "/data/auth-proxy.db"),
+	}
+}
+
+func getEnv(key, defaultValue string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return defaultValue
+}
+
+func parseDuration(key string, defaultVal time.Duration) time.Duration {
+	if v := os.Getenv(key); v != "" {
+		d, err := time.ParseDuration(v)
+		if err == nil {
+			return d
+		}
+	}
+	return defaultVal
+}