Bénévole : retrait de la taille de t-shirt et du régime alimentaire

Champs `tshirt_size` et `diet` supprimés du modèle `Volunteer`, des schémas
d'API, de l'import CSV/Excel (mapping, suggestions, exécution) et du jeu de
démonstration. Énum `TshirtSize` et helper `parse_tshirt_size` supprimés.
Migration 0005. Clé i18n `import.row.invalid_tshirt_size` retirée.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017uUsRg7XzopkCPWimwg9ZM

backend/tests/test_import_service.py

@@ -21,7 +21,6 @@ def _mapping(dry_run: bool = False) -> ImportMapping:
             first_name="Prénom",
             last_name="Nom",
             email="Email",
-            tshirt_size="Taille",
             skills="Compétences",
         ),
         availability_columns=[
@@ -78,7 +77,6 @@ def test_happy_path_creates_volunteers_skills_and_availabilities(session, editio
 
     volunteers = {v.email: v for v in _volunteers(session, edition)}
     assert set(volunteers) == {"alex@ex.fr", "sam@ex.fr"}
-    assert volunteers["alex@ex.fr"].tshirt_size.value == "M"
     assert {s.code for s in volunteers["alex@ex.fr"].skills} == {"SST", "PERMIS_B"}
 
     avails = session.exec(
@@ -194,18 +192,6 @@ def test_invalid_email_is_skipped(session, edition):
     assert report.rows[0].issues[0].code == "import.row.invalid_email"
 
 
-def test_unknown_tshirt_size_warns_and_still_imports(session, edition):
-    report = run_import(
-        session=session,
-        edition=edition,
-        table=_table([["Alex", "Martin", "a@ex.fr", "Large", "", "x", ""]]),
-        mapping=_mapping(),
-    )
-    assert report.created == 1
-    assert report.rows[0].issues[0].code == "import.row.invalid_tshirt_size"
-    assert _volunteers(session, edition)[0].tshirt_size is None
-
-
 def test_unknown_availability_value_is_flagged(session, edition):
     report = run_import(
         session=session,