refactor: remove dead assistant_chat system, consolidate image helpers

The old /assistant/chats/* CRUD endpoints and assistant_chat_service
chat functions were unused — the frontend exclusively uses
/ai-sessions/{id}/chat (unified_chat_service) for all chat operations.

Removed:
- Chat CRUD endpoints (create, list, get, send, delete, conclude)
- assistant_chat_service: create_chat, send_message,
  generate_conclusion_summary, CONCLUSION_SYSTEM_PROMPT
- Frontend: assistantChatApi chat methods, dead types
  (AssistantChat, AssistantChatMessage, ConcludeChatRequest, etc.)

Kept:
- /assistant/retention endpoints (used by ChatRetentionSettingsPage)
- Shared AI infrastructure (_call_ai, _call_anthropic_cached,
  ASSISTANT_SYSTEM_PROMPT, _auto_title) — imported by unified_chat_service

Moved:
- fetch_upload_images + resize_image_for_vision → storage_service.py
  (shared location, not tied to dead endpoint)

Also added "Image Analysis" section to system prompt so Claude knows
to describe attached screenshots.

-650 lines of dead code removed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/api/assistantChat.ts

@@ -1,52 +1,13 @@
 import apiClient from './client'
-import type {
-  AssistantChat,
-  ChatListItem,
-  ChatMessageResponse,
-  RetentionSettings,
-  ConcludeChatRequest,
-  ConcludeChatResponse,
-} from '@/types/assistant-chat'
+import type { RetentionSettings } from '@/types/assistant-chat'
 
+/**
+ * Chat retention settings API.
+ *
+ * Note: Chat CRUD methods were removed — the frontend uses aiSessionsApi
+ * for all chat operations. Only retention settings remain on the /assistant prefix.
+ */
 export const assistantChatApi = {
-  async createChat(): Promise<AssistantChat> {
-    const response = await apiClient.post<AssistantChat>('/assistant/chats', {})
-    return response.data
-  },
-
-  async listChats(page = 1, size = 20): Promise<ChatListItem[]> {
-    const response = await apiClient.get<ChatListItem[]>('/assistant/chats', {
-      params: { page, size },
-    })
-    return response.data
-  },
-
-  async getChat(chatId: string): Promise<AssistantChat> {
-    const response = await apiClient.get<AssistantChat>(`/assistant/chats/${chatId}`)
-    return response.data
-  },
-
-  async sendMessage(chatId: string, message: string): Promise<ChatMessageResponse> {
-    const response = await apiClient.post<ChatMessageResponse>(
-      `/assistant/chats/${chatId}/messages`,
-      { message }
-    )
-    return response.data
-  },
-
-  async updateChat(chatId: string, data: { title?: string; pinned?: boolean }): Promise<AssistantChat> {
-    const response = await apiClient.patch<AssistantChat>(`/assistant/chats/${chatId}`, data)
-    return response.data
-  },
-
-  async deleteChat(chatId: string): Promise<void> {
-    await apiClient.delete(`/assistant/chats/${chatId}`)
-  },
-
-  async bulkDeleteChats(olderThanDays: number): Promise<void> {
-    await apiClient.delete('/assistant/chats', { params: { older_than_days: olderThanDays } })
-  },
-
   async getRetentionSettings(): Promise<RetentionSettings> {
     const response = await apiClient.get<RetentionSettings>('/assistant/retention')
     return response.data
@@ -56,14 +17,6 @@ export const assistantChatApi = {
     const response = await apiClient.patch<RetentionSettings>('/assistant/retention', data)
     return response.data
   },
-
-  async concludeChat(chatId: string, data: ConcludeChatRequest): Promise<ConcludeChatResponse> {
-    const response = await apiClient.post<ConcludeChatResponse>(
-      `/assistant/chats/${chatId}/conclude`,
-      data
-    )
-    return response.data
-  },
 }
 
 export default assistantChatApi