feat(fase2): add Flow schemas and Flow Engine core

API Gateway:
- Add Pydantic schemas for Flow API (create, update, response, nodes, edges)

Flow Engine:
- Add FlowContext for variable management and interpolation
- Add FlowEngine for executing chatbot flows
- Support node types: trigger, message, buttons, wait_input, condition, set_variable

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

services/flow-engine/app/context.py

@@ -0,0 +1,67 @@
+from typing import Any, Optional
+from datetime import datetime
+import re
+
+
+class FlowContext:
+    """Manages variables and state during flow execution"""
+
+    def __init__(
+        self,
+        contact: dict,
+        conversation: dict,
+        message: dict,
+        session_vars: dict = None
+    ):
+        self.contact = contact
+        self.conversation = conversation
+        self.message = message
+        self.variables = session_vars or {}
+        self._system = {
+            "date": datetime.now().strftime("%Y-%m-%d"),
+            "time": datetime.now().strftime("%H:%M"),
+            "day_of_week": datetime.now().strftime("%A"),
+        }
+
+    def get(self, key: str) -> Any:
+        """Get variable by dot notation: contact.name, variables.email"""
+        parts = key.split(".")
+
+        if parts[0] == "contact":
+            return self._get_nested(self.contact, parts[1:])
+        elif parts[0] == "conversation":
+            return self._get_nested(self.conversation, parts[1:])
+        elif parts[0] == "message":
+            return self._get_nested(self.message, parts[1:])
+        elif parts[0] == "system":
+            return self._get_nested(self._system, parts[1:])
+        elif parts[0] == "variables":
+            return self._get_nested(self.variables, parts[1:])
+        else:
+            return self.variables.get(key)
+
+    def set(self, key: str, value: Any):
+        """Set a session variable"""
+        self.variables[key] = value
+
+    def _get_nested(self, obj: dict, keys: list) -> Any:
+        for key in keys:
+            if isinstance(obj, dict):
+                obj = obj.get(key)
+            else:
+                return None
+        return obj
+
+    def interpolate(self, text: str) -> str:
+        """Replace {{variable}} with actual values"""
+        pattern = r'\{\{([^}]+)\}\}'
+
+        def replace(match):
+            key = match.group(1).strip()
+            value = self.get(key)
+            return str(value) if value is not None else ""
+
+        return re.sub(pattern, replace, text)
+
+    def to_dict(self) -> dict:
+        return self.variables.copy()