feat(phase2): add Flow Builder UI and internal flow routes

- Add FlowBuilder.tsx with React Flow visual editor
- Add FlowList.tsx for flow management
- Add /internal/flow/send endpoint for flow-engine messaging
- Add reactflow dependency to frontend

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

services/api-gateway/app/routers/whatsapp.py

@@ -3,6 +3,7 @@ from sqlalchemy.orm import Session
 from typing import List
 from uuid import UUID
 import httpx
+from pydantic import BaseModel
 from app.core.database import get_db
 from app.core.config import get_settings
 from app.core.security import get_current_user
@@ -273,3 +274,59 @@ async def handle_whatsapp_event(
 
     db.commit()
     return {"status": "ok"}
+
+
+class FlowSendRequest(BaseModel):
+    conversation_id: str
+    content: str
+    type: str = "text"
+
+
+@router.post("/internal/flow/send")
+async def flow_send_message(
+    request: FlowSendRequest,
+    db: Session = Depends(get_db),
+):
+    """Internal endpoint for flow engine to send messages"""
+    conversation = db.query(Conversation).filter(
+        Conversation.id == request.conversation_id
+    ).first()
+
+    if not conversation:
+        raise HTTPException(status_code=404, detail="Conversation not found")
+
+    # Create message in DB
+    message = Message(
+        conversation_id=conversation.id,
+        direction=MessageDirection.OUTBOUND,
+        type=MessageType.TEXT,
+        content=request.content,
+        status=MessageStatus.PENDING,
+    )
+    db.add(message)
+    db.commit()
+    db.refresh(message)
+
+    # Send via WhatsApp Core
+    async with httpx.AsyncClient() as client:
+        try:
+            response = await client.post(
+                f"{settings.WHATSAPP_CORE_URL}/api/sessions/{conversation.whatsapp_account_id}/messages",
+                json={
+                    "to": conversation.contact.phone_number,
+                    "type": "text",
+                    "content": {"text": request.content},
+                },
+                timeout=30,
+            )
+            if response.status_code == 200:
+                data = response.json()
+                message.whatsapp_message_id = data.get("messageId")
+                message.status = MessageStatus.SENT
+            else:
+                message.status = MessageStatus.FAILED
+        except Exception:
+            message.status = MessageStatus.FAILED
+
+    db.commit()
+    return {"success": True, "message_id": str(message.id)}