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/types/assistant-chat.ts

@@ -1,20 +1,3 @@
-import type { SuggestedFlow } from './copilot'
-
-export interface AssistantChat {
-  id: string
-  title: string
-  messages: AssistantChatMessage[]
-  message_count: number
-  pinned: boolean
-  created_at: string
-  updated_at: string
-}
-
-export interface AssistantChatMessage {
-  role: 'user' | 'assistant'
-  content: string
-}
-
 export interface ChatListItem {
   id: string
   title: string
@@ -24,27 +7,9 @@ export interface ChatListItem {
   updated_at: string
 }
 
-export interface ChatMessageResponse {
-  content: string
-  suggested_flows: SuggestedFlow[]
-}
-
 export interface RetentionSettings {
   chat_retention_days: number | null
   chat_retention_max_count: number | null
 }
 
 export type ConclusionOutcome = 'resolved' | 'escalated' | 'paused'
-
-export interface ConcludeChatRequest {
-  outcome: ConclusionOutcome
-  notes?: string
-}
-
-export interface ConcludeChatResponse {
-  summary: string
-  outcome: ConclusionOutcome
-  concluded_at: string
-}
-
-export type { SuggestedFlow }

frontend/src/types/index.ts

@@ -11,7 +11,7 @@ export type { Account, Subscription, PlanLimits, SubscriptionDetails, AccountInv
 export * from './admin'
 export * from './analytics'
 export * from './copilot'
-export type { AssistantChat, AssistantChatMessage, ChatListItem, ChatMessageResponse, RetentionSettings } from './assistant-chat'
+export type { ChatListItem, RetentionSettings, ConclusionOutcome } from './assistant-chat'
 export * from './ai-session'
 export * from './flow-proposal'
 export * from './flowpilot-analytics'