feat(server): Build Server-Side Graph Layout Engine (Sub-task 3.4)

[archittmittal]

Description

Implement layout algorithms on the server to structure DAG nodes.
Part of parent issue: #3

Tasks

• Parse hierarchy fields of events in graph_layout.py.
• Calculate tree hierarchies and X/Y coordinates using hierarchical DAG routing.
• Return payload mapped to standard React Flow node positions.

Acceptance Criteria

• Layout computation completes in under 2 seconds for up to 500 nodes.
• Layouts are completely deterministic and client-agnostic.