Pipeline groupé selon `group_by` (commercial|stage|client).

Pipeline groupé selon group_by (commercial|stage|client).

Réponse :
    {
      "group_by": "commercial",
      "period_days": 30,
      "groups": [
        {
          "group_key": "<commercial_id>",
          "group_label": "Alice Dupont",
          "total_amount": 12500.0,
          "currency": "EUR",
          "count_total": 5,
          "stages": {
            "prospect": {"count": 2, "amount": 5000},
            "qualified": {"count": 2, "amount": 5000},
            "won": {"count": 1, "amount": 2500},
            ...
          }
        },
        ...
      ]
    }

Permissions requises : crm:read