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>

src/backup/rotation.py

@@ -0,0 +1,49 @@
+from collections.abc import Iterable
+from datetime import datetime, timedelta
+
+# (période, horizon) pour les paliers à période fixe
+_FIXED_TIERS = (
+    (timedelta(hours=1), timedelta(hours=12)),  # horaire / 12 h
+    (timedelta(hours=12), timedelta(hours=72)),  # 12 h / 72 h
+    (timedelta(days=1), timedelta(days=21)),  # quotidien / 21 j
+)
+_MONTHLY_HORIZON = 12  # mois calendaires
+
+
+def _select_fixed(timestamps, now, period, horizon):
+    cutoff = now - horizon
+    buckets: dict[int, datetime] = {}
+    for t in timestamps:
+        if t > now or t < cutoff:
+            continue
+        idx = int((now - t) // period)
+        if idx not in buckets or t > buckets[idx]:
+            buckets[idx] = t
+    return set(buckets.values())
+
+
+def _month_index(d: datetime) -> int:
+    return d.year * 12 + (d.month - 1)
+
+
+def _select_monthly(timestamps, now):
+    now_idx = _month_index(now)
+    buckets: dict[int, datetime] = {}
+    for t in timestamps:
+        if t > now:
+            continue
+        idx = _month_index(t)
+        if idx > now_idx or now_idx - idx >= _MONTHLY_HORIZON:
+            continue
+        if idx not in buckets or t > buckets[idx]:
+            buckets[idx] = t
+    return set(buckets.values())
+
+
+def select_retained(timestamps: Iterable[datetime], now: datetime) -> set[datetime]:
+    items = list(timestamps)
+    retained: set[datetime] = set()
+    for period, horizon in _FIXED_TIERS:
+        retained |= _select_fixed(items, now, period, horizon)
+    retained |= _select_monthly(items, now)
+    return retained