feat(session): support opt-in Secure session cookies

- add SESSION_COOKIE_SECURE / SESSION_COOKIE_TRUSTED_URL env vars with
  startup validation: enabling Secure requires at least one trusted
  HTTPS entry URL
- wire common.SessionCookieSecure into the session cookie store instead
  of a hardcoded Secure=false
- print a startup warning when Secure session cookies are disabled
- document the new settings in .env.example and docker-compose files

Secure stays off by default because many deployments front new-api with
plain-HTTP reverse proxies, where a hardcoded Secure default would break
logins entirely; enabling it safely depends on the deployment's TLS
setup, so it ships as an opt-in deployment-hardening flag.

middleware/cors.go

@@ -15,7 +15,7 @@ func CORS() gin.HandlerFunc {
 	return cors.New(config)
 }
 
-func PoweredBy() gin.HandlerFunc {
+func Version() gin.HandlerFunc {
 	return func(c *gin.Context) {
 		c.Header("X-New-Api-Version", common.Version)
 		c.Next()