feat: wire image uploads into AI assistant chat (vision support)

- Backend: ChatMessageRequest accepts upload_ids, endpoint fetches
  images from S3, base64-encodes them, passes to Claude as multimodal
  content blocks (vision API)
- Backend: add download_file() to storage_service for fetching from S3
- Frontend: handleSend collects completed upload IDs from pendingUploads
  and includes them in the sendChatMessage API call
- Frontend: prefill handler passes upload IDs from dashboard nav state
- Enables paste-screenshot → AI-sees-it flow end-to-end

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

backend/app/services/assistant_chat_service.py

@@ -87,6 +87,7 @@ async def _call_ai(
     history: list[dict[str, Any]],
     new_message: str,
     max_tokens: int = 4096,
+    images: list[dict[str, Any]] | None = None,
 ) -> tuple[str, int, int]:
     """Call the AI with prompt caching when using Anthropic.
 
@@ -95,13 +96,18 @@ async def _call_ai(
     - RAG context: NOT cached (changes per query)
     - Conversation history prefix: cached via breakpoint on last
       existing message (stable — only new user message is uncached)
+
+    Args:
+        images: Optional list of {"media_type": str, "data": str (base64)}
+                to include alongside the new_message as vision content.
     """
     if settings.AI_PROVIDER == "anthropic" and settings.ANTHROPIC_API_KEY:
         return await _call_anthropic_cached(
-            system_base, rag_context, history, new_message, max_tokens
+            system_base, rag_context, history, new_message, max_tokens,
+            images=images,
         )
 
-    # Fallback: generic provider (Gemini, etc.)
+    # Fallback: generic provider (Gemini, etc.) — images not supported
     from app.core.ai_provider import get_ai_provider
 
     system_prompt = system_base + rag_context
@@ -120,6 +126,7 @@ async def _call_anthropic_cached(
     history: list[dict[str, Any]],
     new_message: str,
     max_tokens: int,
+    images: list[dict[str, Any]] | None = None,
 ) -> tuple[str, int, int]:
     """Call Anthropic with prompt caching on system prompt and history.
 
@@ -168,7 +175,22 @@ async def _call_anthropic_cached(
         }
 
     # Add the new user message (uncached — it's new each turn)
-    messages.append({"role": "user", "content": new_message})
+    # If images are attached, build multimodal content blocks
+    if images:
+        content_blocks: list[dict[str, Any]] = []
+        for img in images:
+            content_blocks.append({
+                "type": "image",
+                "source": {
+                    "type": "base64",
+                    "media_type": img["media_type"],
+                    "data": img["data"],
+                },
+            })
+        content_blocks.append({"type": "text", "text": new_message})
+        messages.append({"role": "user", "content": content_blocks})
+    else:
+        messages.append({"role": "user", "content": new_message})
 
     # MCP server config (optional — controlled by settings)
     mcp_servers = anthropic.NOT_GIVEN
@@ -386,9 +408,14 @@ async def send_message(
     account_id: UUID,
     message: str,
     db: AsyncSession,
+    images: list[dict[str, Any]] | None = None,
 ) -> tuple[str, list[dict[str, Any]], AssistantChat]:
     """Send a user message and get AI response.
 
+    Args:
+        images: Optional list of {"media_type": str, "data": str (base64)}
+                for vision content attached to this message.
+
     Returns (ai_content, suggested_flows, chat).
     """
     result = await db.execute(
@@ -427,6 +454,7 @@ async def send_message(
         rag_context=rag_context,
         history=ai_messages,
         new_message=message,
+        images=images,
     )
 
     # Update chat

backend/app/services/storage_service.py

@@ -67,6 +67,14 @@ async def upload_file(
     return storage_key
 
 
+def download_file(storage_key: str) -> bytes:
+    """Download a file from S3 and return its contents as bytes."""
+    client = _get_client()
+    buf = BytesIO()
+    client.download_fileobj(settings.STORAGE_BUCKET_NAME, storage_key, buf)
+    return buf.getvalue()
+
+
 def get_presigned_url(storage_key: str) -> str:
     """Generate a time-limited presigned URL for downloading a file."""
     client = _get_client()