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/schemas/assistant_chat.py

@@ -1,54 +1,11 @@
-"""Pydantic schemas for standalone AI assistant chat."""
-from typing import Optional, Any, Literal
-from uuid import UUID
-from datetime import datetime
+"""Pydantic schemas for chat retention settings.
+
+Chat CRUD schemas were removed — the active chat system uses
+schemas from ai_session.py via the /ai-sessions endpoints.
+"""
+from typing import Optional
 from pydantic import BaseModel, Field
 
-from app.schemas.copilot import SuggestedFlow
-
-
-class ChatCreateRequest(BaseModel):
-    """Empty body — creates a new blank conversation."""
-    pass
-
-
-class ChatMessageRequest(BaseModel):
-    message: str = Field(..., min_length=1, max_length=8000)
-    upload_ids: list[UUID] = Field(default_factory=list, max_length=10)
-
-
-class ChatMessageResponse(BaseModel):
-    content: str
-    suggested_flows: list[SuggestedFlow] = []
-
-
-class ChatListResponse(BaseModel):
-    id: UUID
-    title: str
-    message_count: int
-    pinned: bool
-    created_at: datetime
-    updated_at: datetime
-
-    model_config = {"from_attributes": True}
-
-
-class ChatDetailResponse(BaseModel):
-    id: UUID
-    title: str
-    messages: list[dict[str, Any]]
-    message_count: int
-    pinned: bool
-    created_at: datetime
-    updated_at: datetime
-
-    model_config = {"from_attributes": True}
-
-
-class ChatUpdateRequest(BaseModel):
-    title: Optional[str] = Field(None, min_length=1, max_length=255)
-    pinned: Optional[bool] = None
-
 
 class RetentionSettingsResponse(BaseModel):
     chat_retention_days: Optional[int]
@@ -58,14 +15,3 @@ class RetentionSettingsResponse(BaseModel):
 class RetentionSettingsUpdate(BaseModel):
     chat_retention_days: Optional[int] = Field(None, ge=1, le=365)
     chat_retention_max_count: Optional[int] = Field(None, ge=10, le=10000)
-
-
-class ConcludeChatRequest(BaseModel):
-    outcome: Literal["resolved", "escalated", "paused"]
-    notes: Optional[str] = Field(None, max_length=2000)
-
-
-class ConcludeChatResponse(BaseModel):
-    summary: str
-    outcome: str
-    concluded_at: datetime