feat(backup): snapshot SQLite cohérent et contrôle d'intégrité (#89)

Implémente les trois fonctions de gestion de snapshots SQLite:
- make_snapshot: crée un snapshot gzippé cohérent via sqlite3.Connection.backup()
- write_snapshot: écrit le snapshot décompressé à destination
- verify_integrity: valide l'intégrité d'une base avec PRAGMA integrity_check

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

tests/backup/test_snapshot.py

@@ -0,0 +1,36 @@
+import sqlite3
+from pathlib import Path
+
+from src.backup.snapshot import make_snapshot, verify_integrity, write_snapshot
+
+
+def _make_db(path: Path):
+    conn = sqlite3.connect(str(path))
+    conn.execute("CREATE TABLE t (id INTEGER, val TEXT)")
+    conn.execute("INSERT INTO t VALUES (1, 'a'), (2, 'b')")
+    conn.commit()
+    conn.close()
+
+
+def test_snapshot_roundtrip_preserves_rows(tmp_path):
+    src = tmp_path / "users.sqlite"
+    _make_db(src)
+    gzipped = make_snapshot(src)
+    restored = tmp_path / "restored.sqlite"
+    write_snapshot(gzipped, restored)
+    conn = sqlite3.connect(str(restored))
+    rows = conn.execute("SELECT id, val FROM t ORDER BY id").fetchall()
+    conn.close()
+    assert rows == [(1, "a"), (2, "b")]
+
+
+def test_verify_integrity_ok(tmp_path):
+    src = tmp_path / "users.sqlite"
+    _make_db(src)
+    assert verify_integrity(src) is True
+
+
+def test_verify_integrity_detects_corruption(tmp_path):
+    bad = tmp_path / "bad.sqlite"
+    bad.write_bytes(b"this is not a sqlite database")
+    assert verify_integrity(bad) is False