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/base_controller.rb

@@ -266,4 +266,11 @@ class Api::V1::BaseController < ApplicationController
         end
       end
     end
+
+    # Check if AI features are enabled for the current user
+    def require_ai_enabled
+      unless current_resource_owner&.ai_enabled?
+        render_json({ error: "feature_disabled", message: "AI features are not enabled for this user" }, status: :forbidden)
+      end
+    end
 end