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/yt_backend/connection_pool.cr

@@ -63,7 +63,7 @@ struct CompanionConnectionPool
     end
   end
 
-  def client(&)
+  def client(env : HTTP::Server::Context | Nil, &)
     conn = pool.checkout
 
     begin
@@ -71,7 +71,13 @@ struct CompanionConnectionPool
     rescue ex
       conn.close
 
-      companion = CONFIG.invidious_companion.sample
+      if env.nil?
+        companion = CONFIG.invidious_companion.sample
+      else
+        current_companion = env.get("current_companion").as(Int32)
+        companion = CONFIG.invidious_companion[current_companion]
+      end
+
       conn = make_client(companion.private_url, use_http_proxy: false)
 
       response = yield conn

src/invidious/yt_backend/youtube_api.cr

@@ -456,6 +456,7 @@ module YoutubeAPI
     *, # Force the following parameters to be passed by name
     params : String,
     client_config : ClientConfig | Nil = nil,
+    env : HTTP::Server::Context | Nil = nil,
   )
     # Playback context, separate because it can be different between clients
     playback_ctx = {
@@ -492,7 +493,7 @@ module YoutubeAPI
     end
 
     if CONFIG.invidious_companion.present?
-      return self._post_invidious_companion("/youtubei/v1/player", data)
+      return self._post_invidious_companion("/youtubei/v1/player", data, env)
     else
       return self._post_json("/youtubei/v1/player", data, client_config)
     end
@@ -673,6 +674,7 @@ module YoutubeAPI
   def _post_invidious_companion(
     endpoint : String,
     data : Hash,
+    env : HTTP::Server::Context | Nil,
   ) : Hash(String, JSON::Any)
     headers = HTTP::Headers{
       "Content-Type"  => "application/json; charset=UTF-8",
@@ -686,7 +688,7 @@ module YoutubeAPI
     # Send the POST request
 
     begin
-      response = COMPANION_POOL.client &.post(endpoint, headers: headers, body: data.to_json)
+      response = COMPANION_POOL.client(env, &.post(endpoint, headers: headers, body: data.to_json))
       body = response.body
       if (response.status_code != 200)
         raise Exception.new(