fix(arch): l inscription est une operation, donc une transaction

ARCH-006, avec une requalification du constat.

**Le chiffre de l audit surestimait le probleme.** « 58 commit() en routes
pour un seul rollback() » lisait un ratio comme un defaut. Mesure plutot
que suppose :

  - Flask-SQLAlchemy demonte la session a la fin de chaque requete, ce qui
    annule tout ce qui n a pas ete commite ;
  - l unique rollback est dans le gestionnaire 500, c est-a-dire au bon
    endroit ;
  - depuis la vague D, aucun module de routes ne contient `except
    Exception` : les 34 releves sont dans discord_bot.py, les scripts et le
    service de notification, ou avaler l erreur est le comportement voulu
    et documente.

Ce que le decompte ne pouvait pas voir, c est le vrai defaut : une fonction
qui commite DEUX fois, ou un echec apres le premier commit laisse une
demi-operation persistee. Il y en avait deux dans tout le depot -- une
analyse AST le confirme. edit_user etait la grave, corrigee avec ARCH-008.

register est la seconde : le compte etait commite, puis les gamertags dans
une seconde transaction. Un echec entre les deux laissait un compte dont
les jeux declares etaient absents, l inscription etant annoncee reussie.
Le premier commit devient un flush -- l identifiant est necessaire pour les
lignes suivantes, pas la durabilite.

Les deux commit() de login ne sont pas concernes : ils sont dans des
branches mutuellement exclusives, succes et echec.

tests/test_transactions.py enonce la garantie plutot que de la supposer :
un echec en cours de requete ne laisse aucune ligne, et l inscription est
tout ou rien. Le second echoue sur le code d avant.

Co-Authored-By: Claude Opus 5 <[email protected]>

tests/test_transactions.py

@@ -0,0 +1,111 @@
+"""What survives a failure mid-request — ARCH-006.
+
+The audit counted 58 `db.session.commit()` in the routes against a single
+`db.session.rollback()` in the whole repository, and read that ratio as a
+problem. Measured rather than assumed, it mostly is not one:
+
+  - Flask-SQLAlchemy tears the session down at the end of every request,
+    which rolls back anything uncommitted;
+  - the one rollback is in the 500 handler, which is where it belongs;
+  - after the wave-D work, no route module contains `except Exception` at
+    all, so nothing swallows a failure and carries on.
+
+What the count could not see is the real defect: a route that commits
+*twice*, so a failure after the first one leaves half an operation
+persisted. There were two such functions. `edit_user` was the serious one
+and was fixed with ARCH-008; `register` is fixed here.
+
+These tests state the guarantee, so that removing the rollback or
+reintroducing a mid-operation commit fails loudly.
+"""
+
+import pytest
+
+from app.models import User, UserGamertag
+
+
+class TestNothingPartialSurvives:
+    def test_a_failure_after_add_leaves_no_row(self, app, client, as_role, monkeypatch):
+        """The session is torn down per request; an uncommitted add is gone."""
+        as_role('admin')
+
+        from app.routes.users import accounts
+
+        def _explode(*args, **kwargs):
+            raise RuntimeError('storage unavailable')
+
+        monkeypatch.setattr(accounts.db.session, 'commit', _explode)
+
+        with pytest.raises(RuntimeError):
+            client.post(
+                '/users/create',
+                data={
+                    'username': 'ghost',
+                    'email': '[email protected]',
+                    'password': 'Password123',
+                    'full_name': 'Ghost Account',
+                    'role': 'coach',
+                },
+            )
+
+        with app.app_context():
+            assert User.query.filter_by(username='ghost').first() is None
+
+    def test_the_error_handler_still_rolls_back(self, app):
+        """The single rollback in the repository is the one in the 500
+        handler. Removing it should break something visible."""
+        import inspect
+
+        from app import app as app_module
+
+        source = inspect.getsource(app_module.create_app)
+        assert 'db.session.rollback()' in source
+
+
+class TestRegistrationIsOneOperation:
+    """register() committed the account, then committed the gamertags. A
+    failure in between left an account whose declared games were absent,
+    with the sign-up reported as successful."""
+
+    FORM = {
+        'username': 'newplayer',
+        'email': '[email protected]',
+        'password': 'Password123',
+        'confirm_password': 'Password123',
+        'full_name': 'New Player',
+        'games': 'Valorant',
+        'gamertag_Valorant': 'newplayer#1234',
+    }
+
+    def _submit(self, client, app, **overrides):
+        with client.session_transaction() as session:
+            session['captcha_answer'] = 4
+        payload = dict(self.FORM, captcha_answer='4')
+        payload.update(overrides)
+        return client.post('/auth/register', data=payload, follow_redirects=True)
+
+    def test_a_successful_sign_up_stores_both(self, app, client):
+        self._submit(client, app)
+
+        with app.app_context():
+            user = User.query.filter_by(username='newplayer').one()
+            tags = UserGamertag.query.filter_by(user_id=user.id).all()
+            assert [tag.gamertag for tag in tags] == ['newplayer#1234']
+
+    def test_a_failure_leaves_no_half_account(self, app, client, monkeypatch):
+        from app.routes import auth as auth_module
+
+        real_add = auth_module.db.session.add
+
+        def _explode_on_gamertag(instance, *args, **kwargs):
+            if isinstance(instance, UserGamertag):
+                raise RuntimeError('storage unavailable')
+            return real_add(instance, *args, **kwargs)
+
+        monkeypatch.setattr(auth_module.db.session, 'add', _explode_on_gamertag)
+
+        with pytest.raises(RuntimeError):
+            self._submit(client, app)
+
+        with app.app_context():
+            assert User.query.filter_by(username='newplayer').first() is None