feat(fase2): add Flow API routes and Flow Engine main app

API Gateway:
- Add flows router with CRUD endpoints
- Add activate/deactivate endpoints
- Auto-deactivate other flows for welcome/fallback triggers

Flow Engine:
- Add main.py with FastAPI app
- Add /process endpoint for message handling
- Add SQLAlchemy models (mirrors api-gateway)

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

services/flow-engine/app/main.py

@@ -0,0 +1,49 @@
+from fastapi import FastAPI, Depends
+from pydantic import BaseModel
+from typing import Optional
+from sqlalchemy.orm import Session
+from app.models import get_db
+from app.engine import FlowEngine
+from app.config import get_settings
+
+settings = get_settings()
+
+app = FastAPI(
+    title="WhatsApp Centralizado - Flow Engine",
+    version="1.0.0",
+)
+
+
+class ProcessMessageRequest(BaseModel):
+    conversation_id: str
+    contact: dict
+    conversation: dict
+    message: dict
+
+
+class ProcessMessageResponse(BaseModel):
+    handled: bool
+    flow_id: Optional[str] = None
+
+
+@app.get("/health")
+def health_check():
+    return {"status": "ok", "service": "flow-engine"}
+
+
+@app.post("/process", response_model=ProcessMessageResponse)
+async def process_message(
+    request: ProcessMessageRequest,
+    db: Session = Depends(get_db),
+):
+    """Process an incoming message through the flow engine"""
+    engine = FlowEngine(db)
+
+    handled = await engine.process_message(
+        conversation_id=request.conversation_id,
+        contact=request.contact,
+        conversation=request.conversation,
+        message=request.message,
+    )
+
+    return ProcessMessageResponse(handled=handled)