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/chats/_chat.json.jbuilder

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+json.id chat.id
+json.title chat.title
+json.error chat.error.present? ? chat.error : nil
+json.created_at chat.created_at.iso8601
+json.updated_at chat.updated_at.iso8601

app/views/api/v1/chats/index.json.jbuilder

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+json.chats @chats do |chat|
+  json.id chat.id
+  json.title chat.title
+  json.last_message_at chat.messages.ordered.first&.created_at&.iso8601
+  json.message_count chat.messages.count
+  json.error chat.error.present? ? chat.error : nil
+  json.created_at chat.created_at.iso8601
+  json.updated_at chat.updated_at.iso8601
+end
+
+json.pagination do
+  json.page @pagy.page
+  json.per_page @pagy.vars[:items]
+  json.total_count @pagy.count
+  json.total_pages @pagy.pages
+end

app/views/api/v1/chats/show.json.jbuilder

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+json.partial! "chat", chat: @chat
+
+json.messages @messages do |message|
+  json.id message.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
+  
+  # Include tool calls for assistant messages
+  if message.type == "AssistantMessage" && message.tool_calls.any?
+    json.tool_calls message.tool_calls do |tool_call|
+      json.id tool_call.id
+      json.function_name tool_call.function_name
+      json.function_arguments tool_call.function_arguments
+      json.function_result tool_call.function_result
+      json.created_at tool_call.created_at.iso8601
+    end
+  end
+end
+
+if @pagy
+  json.pagination do
+    json.page @pagy.page
+    json.per_page @pagy.vars[:items]
+    json.total_count @pagy.count
+    json.total_pages @pagy.pages
+  end
+end

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