v0.8.0: Model selection, thinking effort and conversation rating (0.8)

ModelSelection lists the models and effort levels a user may pick for a
conversation. The choice is stored on the conversation and sent as a
per-session override of the agent (agent_with_overrides); it is locked after
the first message, because a session cannot switch models. The context gauge
uses the window of the chosen model.

The owner can rate a conversation (thumbs up / down + an optional comment) via
ChatService::rateConversation() and ChatEndpoint::rate(); a read-only view
shows the rating.

The widget gets the model and effort selects, the rating box and `params`
(+ setParams()) for extra request parameters.

BC break: ManagedAgentsClient::createSession() has a 4th parameter $model,
ConversationInterface new accessors and the conversation table five new
columns - see "Upgrading to 0.8".