Add CSV export for graph statistics

[dentity007]

What needs to be done

Add a GET /api/graph/stats/export endpoint that returns graph statistics (node counts, edge counts, expert rankings) as a downloadable CSV file.

Why this matters

Researchers want to analyze their knowledge graph metrics in Excel/Sheets.

Where to look

• forge/api/routes/graph.py (add new endpoint)
• forge/graph/engine.py (get_stats already exists)

How to test

pytest tests/test_api_graph.py -v

Acceptance criteria

• New endpoint returns valid CSV
• Tests added for the new endpoint
• Existing tests still pass