feat: add session-to-flow request/response schemas (Task 19)

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

backend/app/schemas/session_to_flow.py

@@ -0,0 +1,13 @@
+from pydantic import BaseModel
+
+
+class SessionToFlowRequest(BaseModel):
+    session_id: str
+
+
+class SessionToFlowResponse(BaseModel):
+    name: str
+    description: str
+    tree_type: str  # Always "procedural"
+    tags: list[str]
+    tree_structure: dict  # Procedural steps with optional fallback_steps