Add API v1 chat endpoints

- Add chats#index and chats#show endpoints to list and view AI conversations
- Add messages#create endpoint to send messages to AI chats
- Include API documentation for chat endpoints
- Add controller tests for new endpoints

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/controllers/api/v1/messages_controller.rb

@@ -0,0 +1,55 @@
+# frozen_string_literal: true
+
+class Api::V1::MessagesController < Api::V1::BaseController
+  before_action :require_ai_enabled
+  before_action :ensure_write_scope, only: [ :create, :retry ]
+  before_action :set_chat
+
+  def create
+    @message = @chat.messages.build(
+      content: message_params[:content],
+      type: "UserMessage",
+      ai_model: message_params[:model] || "gpt-4"
+    )
+
+    if @message.save
+      AssistantResponseJob.perform_later(@message)
+      render :show, status: :created
+    else
+      render json: { error: "Failed to create message", details: @message.errors.full_messages }, status: :unprocessable_entity
+    end
+  end
+
+  def retry
+    last_message = @chat.messages.ordered.last
+
+    if last_message&.type == "AssistantMessage"
+      new_message = @chat.messages.create!(
+        type: "AssistantMessage",
+        content: "",
+        ai_model: last_message.ai_model
+      )
+
+      AssistantResponseJob.perform_later(new_message)
+      render json: { message: "Retry initiated", message_id: new_message.id }, status: :accepted
+    else
+      render json: { error: "No assistant message to retry" }, status: :unprocessable_entity
+    end
+  end
+
+  private
+
+    def ensure_write_scope
+      authorize_scope!(:write)
+    end
+
+    def set_chat
+      @chat = Current.user.chats.find(params[:chat_id])
+    rescue ActiveRecord::RecordNotFound
+      render json: { error: "Chat not found" }, status: :not_found
+    end
+
+    def message_params
+      params.permit(:content, :model)
+    end
+end