feat(backup): nommage horodaté des clés S3 (#89)

Ajoute les fonctions make_key() et parse_timestamp() pour gérer
le format des clés S3 avec horodatage UTC.

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

tests/backup/test_naming.py

@@ -0,0 +1,25 @@
+from datetime import datetime, timezone
+
+import pytest
+
+from src.backup.naming import make_key, parse_timestamp
+
+
+def test_make_key_format():
+    ts = datetime(2026, 6, 24, 14, 5, 9, tzinfo=timezone.utc)
+    assert make_key("backups", ts) == "backups/users-20260624T140509Z.sqlite.gz.enc"
+
+
+def test_make_key_strips_trailing_slash():
+    ts = datetime(2026, 6, 24, 14, 5, 9, tzinfo=timezone.utc)
+    assert make_key("backups/", ts).startswith("backups/users-")
+
+
+def test_roundtrip():
+    ts = datetime(2026, 1, 2, 3, 4, 5, tzinfo=timezone.utc)
+    assert parse_timestamp(make_key("p", ts)) == ts
+
+
+def test_parse_rejects_unknown_key():
+    with pytest.raises(ValueError):
+        parse_timestamp("backups/users.sqlite.bak")