feat: add PostgreSQL persistence layer with Neon DB support

internal/config/config.go

@@ -13,6 +13,7 @@ const (
 
 type Config struct {
 	HTTPAddr        string
+	DatabaseURL     string
 	Environment     string
 	WorkflowRuntime string
 	ModelKey        string
@@ -23,6 +24,7 @@ type Config struct {
 func LoadFromEnv() Config {
 	return Config{
 		HTTPAddr:        envOrDefault("TUTOR_HTTP_ADDR", defaultHTTPAddr),
+		DatabaseURL:     envOrDefault("DATABASE_URL", ""),
 		Environment:     envOrDefault("TUTOR_ENV", defaultEnvironment),
 		WorkflowRuntime: envOrDefault("TUTOR_WORKFLOW_RUNTIME", defaultWorkflowRuntime),
 		ModelKey:        envOrDefault("TUTOR_MODEL_KEY", defaultModelKey),