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>

backend/app/api/endpoints/ai_sessions.py

@@ -283,8 +283,8 @@ async def send_chat_message(
     # Fetch attached images from S3 (if any)
     images = None
     if data.upload_ids:
-        from app.api.endpoints.assistant_chat import _fetch_upload_images
-        images = await _fetch_upload_images(data.upload_ids, account_id, db) or None
+        from app.services.storage_service import fetch_upload_images
+        images = await fetch_upload_images(data.upload_ids, account_id, db) or None
 
     try:
         ai_content, suggested_flows, session = await unified_chat_service.send_chat_message(