fix: JSON serialization crash on NaN floats in cycle context snapshot

- portfolio_context.py: add _safe_float() helper (converts NaN/Inf → None);
  use .squeeze().dropna() on yfinance closes before computing moves;
  guard division by checking closes.iloc[-2] != 0
- cycle.py: add _sanitize_floats() recursive sanitizer applied to the full
  snapshot before FastAPI serializes it — catches any remaining NaN from
  iv_rank, technical indicators, or other sources

Fixes 500 on GET /api/cycle/contexts/{run_id} when yfinance returns NaN
weekend data for portfolio positions.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/routers/cycle.py

@@ -91,13 +91,25 @@ def list_context_snapshots(limit: int = 30):
     return {"snapshots": list_cycle_context_snapshots(limit=limit)}
 
 
+def _sanitize_floats(obj):
+    """Recursively replace NaN/Inf floats with None for JSON-safe serialization."""
+    import math
+    if isinstance(obj, float):
+        return None if (math.isnan(obj) or math.isinf(obj)) else obj
+    if isinstance(obj, dict):
+        return {k: _sanitize_floats(v) for k, v in obj.items()}
+    if isinstance(obj, list):
+        return [_sanitize_floats(v) for v in obj]
+    return obj
+
+
 @router.get("/contexts/{run_id}")
 def get_context_snapshot(run_id: str):
     """Return the full context snapshot for a given cycle run_id."""
     snap = get_cycle_context_snapshot(run_id)
     if not snap:
         raise HTTPException(404, "Snapshot non trouvé pour ce cycle")
-    return snap
+    return _sanitize_floats(snap)
 
 
 @router.get("/ai-calls/{run_id}")