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>

src/backup/snapshot.py

@@ -0,0 +1,40 @@
+import gzip
+import sqlite3
+import tempfile
+from pathlib import Path
+
+
+def make_snapshot(db_path: Path) -> bytes:
+    with tempfile.NamedTemporaryFile(suffix=".sqlite", delete=False) as tmp:
+        tmp_path = Path(tmp.name)
+    try:
+        src = sqlite3.connect(str(db_path))
+        try:
+            dst = sqlite3.connect(str(tmp_path))
+            try:
+                src.backup(dst)
+            finally:
+                dst.close()
+        finally:
+            src.close()
+        return gzip.compress(tmp_path.read_bytes())
+    finally:
+        tmp_path.unlink(missing_ok=True)
+
+
+def write_snapshot(gzipped: bytes, dest: Path) -> None:
+    dest.write_bytes(gzip.decompress(gzipped))
+
+
+def verify_integrity(db_path: Path) -> bool:
+    try:
+        conn = sqlite3.connect(str(db_path))
+    except sqlite3.DatabaseError:
+        return False
+    try:
+        row = conn.execute("PRAGMA integrity_check").fetchone()
+        return row is not None and row[0] == "ok"
+    except sqlite3.DatabaseError:
+        return False
+    finally:
+        conn.close()