Add legacy Fernet migration

src/tests/test_legacy_migration.py

@@ -0,0 +1,43 @@
+import json
+import hashlib
+from pathlib import Path
+
+from helpers import create_vault, TEST_SEED, TEST_PASSWORD
+from utils.key_derivation import derive_index_key
+from cryptography.fernet import Fernet
+
+
+def test_legacy_index_migrates(tmp_path: Path):
+    vault, _ = create_vault(tmp_path, TEST_SEED, TEST_PASSWORD)
+
+    key = derive_index_key(TEST_SEED)
+    data = {
+        "schema_version": 4,
+        "entries": {
+            "0": {
+                "label": "a",
+                "length": 8,
+                "type": "password",
+                "kind": "password",
+                "notes": "",
+                "custom_fields": [],
+                "origin": "",
+                "tags": [],
+            }
+        },
+    }
+    enc = Fernet(key).encrypt(json.dumps(data).encode())
+    legacy_file = tmp_path / "seedpass_passwords_db.json.enc"
+    legacy_file.write_bytes(enc)
+    (tmp_path / "seedpass_passwords_db_checksum.txt").write_text(
+        hashlib.sha256(enc).hexdigest()
+    )
+
+    loaded = vault.load_index()
+    assert loaded == data
+
+    new_file = tmp_path / "seedpass_entries_db.json.enc"
+    assert new_file.exists()
+    assert not legacy_file.exists()
+    assert not (tmp_path / "seedpass_passwords_db_checksum.txt").exists()
+    assert (tmp_path / ("seedpass_entries_db.json.enc.fernet")).exists()