Type de paiement à montant libre (pensé pour le mécénat)

FormPaymentOption.free_amount (migration 0021) : case « Montant libre »
sur l'écran Inscriptions ; le montant configuré devient le minimum et le
bénévole saisit le sien sur le formulaire public (payment_amount, virgule
ou point, 2 décimales max, plafond 10 000 € contre une faute de frappe —
422 form.invalid_free_amount sinon). Validé côté client avant l'envoi,
le serveur restant seul juge. Le montant choisi est celui du checkout
SumUp, recopié sur la fiche, l'email de confirmation et le cerfa.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/tests/test_forms_api.py

@@ -1089,3 +1089,87 @@ def test_pending_status_polling_is_rate_limited_per_token(
         pending_status_limiter.hit(token)
     assert raw_client.get(_form_url(edition, f"/public/pending/{token}")).status_code == 429
 
+
+def _configure_free_amount(client, edition, minimum=10):
+    """Type de paiement « montant libre » : `amount` est le minimum."""
+    client.put(_form_url(edition, "/fields"), json={"fields": ["WILLING_TO_PAY_FEE"]})
+    client.patch(
+        _form_url(edition), json={"sumup_merchant_code": _MERCHANT, "sumup_api_key": _API_KEY}
+    )
+    resp = client.post(
+        _form_url(edition, "/payment-options"),
+        json={"label": "Don libre", "amount": minimum, "free_amount": True},
+    )
+    assert resp.status_code == 201
+    assert resp.json()["free_amount"] is True
+    return resp.json()["id"]
+
+
+def test_free_amount_option_is_exposed_on_the_public_form(raw_client, client, edition):
+    _configure_free_amount(client, edition)
+    config = raw_client.get(_public(edition, "/public")).json()
+    assert config["payment_options"][0]["free_amount"] is True
+    assert config["payment_options"][0]["amount"] == 10
+
+
+def test_free_amount_uses_the_amount_chosen_by_the_volunteer(
+    raw_client, client, edition, monkeypatch
+):
+    option_id = _configure_free_amount(client, edition)
+    checkouts = []
+
+    def fake_checkout(**kw):
+        checkouts.append(kw)
+        return SumUpCheckout(id="co_free", status="PENDING", hosted_checkout_url="https://x")
+
+    monkeypatch.setattr("app.api.forms.create_checkout", fake_checkout)
+    resp = _submit(
+        raw_client, edition,
+        first_name="Alex", last_name="Martin", hcaptcha_token="x",
+        payment_option_id=option_id, payment_amount="75,50",
+    )
+    assert resp.status_code == 201
+    assert checkouts[0]["amount"] == 75.5
+
+    token = resp.json()["pending_token"]
+    _pay(raw_client, edition, monkeypatch, checkout_id="co_free")
+    volunteer_id = _pending_status(raw_client, edition, token)["volunteer_id"]
+    fiche = client.get(f"/editions/{edition.id}/volunteers/{volunteer_id}").json()
+    assert fiche["fee_amount"] == 75.5
+    assert fiche["fee_payment_label"] == "Don libre"
+
+
+@pytest.mark.parametrize("amount", [None, "", "abc", "9.99", "0", "-5", "10000.01", "12.345"])
+def test_free_amount_rejects_missing_or_out_of_range_amounts(
+    raw_client, client, edition, monkeypatch, amount
+):
+    option_id = _configure_free_amount(client, edition)
+    monkeypatch.setattr("app.api.forms.create_checkout", lambda **kw: None)
+    fields = dict(
+        first_name="Alex", last_name="Martin", hcaptcha_token="x", payment_option_id=option_id
+    )
+    if amount is not None:
+        fields["payment_amount"] = amount
+    resp = _submit(raw_client, edition, **fields)
+    assert resp.status_code == 422
+    detail = resp.json()["detail"]
+    assert detail["code"] == "form.invalid_free_amount"
+    assert detail["params"] == {"min": 10.0, "max": 10000.0}
+
+
+def test_fixed_amount_option_ignores_any_amount_sent(raw_client, client, edition, monkeypatch):
+    option_id = _configure_sumup(client, edition, amount=12.5)
+    checkouts = []
+
+    def fake_checkout(**kw):
+        checkouts.append(kw)
+        return SumUpCheckout(id="co_fixed", status="PENDING", hosted_checkout_url="https://x")
+
+    monkeypatch.setattr("app.api.forms.create_checkout", fake_checkout)
+    resp = _submit(
+        raw_client, edition,
+        first_name="Alex", last_name="Martin", hcaptcha_token="x",
+        payment_option_id=option_id, payment_amount="1",
+    )
+    assert resp.status_code == 201
+    assert checkouts[0]["amount"] == 12.5