feat: do all the backend balancing on the invidious side

This will make invidious easier to maintain and escalate without the need of an overcomplicated reverse proxy configuration and multiple invidious instances with each one with a different configuration (in this case, invidious companion)

src/invidious/routes/backend_switcher.cr

@@ -0,0 +1,16 @@
+{% skip_file if flag?(:api_only) %}
+
+module Invidious::Routes::BackendSwitcher
+  def self.switch(env)
+    referer = get_referer(env)
+    backend_id = env.params.query["backend_id"]?.try &.to_i
+
+    if backend_id.nil?
+      return error_template(400, "Backend ID is required")
+    end
+
+    env.response.cookies[CONFIG.server_id_cookie_name] = Invidious::User::Cookies.server_id(env.request.headers["Host"], backend_id)
+
+    env.redirect referer
+  end
+end