feat(backup): rotation multi-paliers (fonction pure)

Implémente la fonction select_retained() pour la rétention multi-paliers:
- Paliers fixes: horaire/12h, 12h/72h, quotidien/21j
- Palier mensuel: 12 mois calendaires
- Fonction pure, pas de dépendances sur d'autres modules backup

Fixes #89
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/backup/test_rotation.py

@@ -0,0 +1,52 @@
+from datetime import datetime, timedelta, timezone
+
+from src.backup.rotation import select_retained
+
+NOW = datetime(2026, 6, 24, 12, 0, 0, tzinfo=timezone.utc)
+
+
+def test_most_recent_always_kept():
+    ts = [NOW, NOW - timedelta(hours=2)]
+    assert NOW in select_retained(ts, NOW)
+
+
+def test_recent_hours_kept_by_hourly_tier():
+    t = NOW - timedelta(hours=3)
+    assert t in select_retained([t], NOW)
+
+
+def test_two_backups_same_hour_keep_newest():
+    older = NOW - timedelta(hours=2, minutes=50)
+    newer = NOW - timedelta(hours=2, minutes=10)
+    retained = select_retained([older, newer], NOW)
+    assert newer in retained
+    assert older not in retained
+
+
+def test_daily_tier_keeps_old_daily_backup():
+    t = NOW - timedelta(days=10)
+    assert t in select_retained([t], NOW)
+
+
+def test_monthly_tier_keeps_one_per_calendar_month():
+    older = NOW - timedelta(days=40)  # mois M-2 ou M-1 selon calendrier
+    same_month_newer = older + timedelta(days=2)
+    retained = select_retained([older, same_month_newer], NOW)
+    # même mois calendaire -> seul le plus récent du mois est conservé
+    assert same_month_newer in retained
+    assert older not in retained
+
+
+def test_keeps_backup_within_12_months():
+    t = NOW - timedelta(days=300)
+    assert t in select_retained([t], NOW)
+
+
+def test_drops_backup_older_than_12_months():
+    t = NOW - timedelta(days=400)
+    assert t not in select_retained([t], NOW)
+
+
+def test_future_timestamps_ignored():
+    t = NOW + timedelta(hours=1)
+    assert t not in select_retained([t], NOW)