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/views/api/v1/messages/show.json.jbuilder

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+json.id @message.id
+json.chat_id @message.chat_id
+json.type @message.type.underscore
+json.role @message.role
+json.content @message.content
+json.model @message.ai_model if @message.type == "AssistantMessage"
+json.created_at @message.created_at.iso8601
+json.updated_at @message.updated_at.iso8601
+
+# Note: AI response will be processed asynchronously
+if @message.type == "UserMessage"
+  json.ai_response_status "pending"
+  json.ai_response_message "AI response is being generated"
+end