feat(fase2): add Flow models, Flow Engine setup, update Docker Compose

- Add Flow and FlowSession SQLAlchemy models for chatbot flows
- Add TriggerType enum (welcome, keyword, fallback, event, manual)
- Setup flow-engine service with FastAPI structure
- Add flow-engine to docker-compose.yml

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

services/api-gateway/app/models/flow.py

@@ -0,0 +1,44 @@
+import uuid
+from datetime import datetime
+from sqlalchemy import Column, String, Boolean, DateTime, Text, Integer, Enum as SQLEnum
+from sqlalchemy.dialects.postgresql import UUID, JSONB
+import enum
+from app.core.database import Base
+
+
+class TriggerType(str, enum.Enum):
+    WELCOME = "welcome"
+    KEYWORD = "keyword"
+    FALLBACK = "fallback"
+    EVENT = "event"
+    MANUAL = "manual"
+
+
+class Flow(Base):
+    __tablename__ = "flows"
+
+    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    name = Column(String(100), nullable=False)
+    description = Column(Text, nullable=True)
+    trigger_type = Column(SQLEnum(TriggerType), nullable=False)
+    trigger_value = Column(String(255), nullable=True)
+    nodes = Column(JSONB, default=list)
+    edges = Column(JSONB, default=list)
+    variables = Column(JSONB, default=dict)
+    is_active = Column(Boolean, default=False, nullable=False)
+    version = Column(Integer, default=1, nullable=False)
+    created_at = Column(DateTime, default=datetime.utcnow, nullable=False)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False)
+
+
+class FlowSession(Base):
+    __tablename__ = "flow_sessions"
+
+    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    conversation_id = Column(UUID(as_uuid=True), nullable=False, index=True)
+    flow_id = Column(UUID(as_uuid=True), nullable=False)
+    current_node_id = Column(String(100), nullable=True)
+    variables = Column(JSONB, default=dict)
+    waiting_for_input = Column(Boolean, default=False)
+    created_at = Column(DateTime, default=datetime.utcnow, nullable=False)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False)