test(api): cover backup restore and path containment, and fix restore scope

Restore is the most destructive thing the web interface can do — it
overwrites config, secrets, WiFi settings and fonts, then reinstalls
plugins — and neither it nor the file routes beside it had tests.

A malformed `options` field fell back to {}. Every RestoreOptions flag
defaults to True, so a caller who asked for a narrow restore and
mis-serialized the request got a full one instead, secrets included, and
was told it succeeded. Valid JSON that is not an object was worse:
`"null"` or `"[1,2]"` reached .get() on a non-dict and raised, so the
request died as a generic 500. Both are now refused with a 400 that says
what was wrong, and restore_backup is never reached.

The other file routes take a filename straight out of the URL and turn it
into a path — one to read, one to unlink. _safe_backup_path is the only
thing keeping those inside the export directory, and it was untested. No
bypass was found; the thirteen traversal shapes are pinned so a later
loosening of that pattern has to argue with something. The delete route's
by-name enumeration is covered too, including that a directory sharing a
backup's name is not removed.

84 tests. Two behaviours are pinned as intentional: a failed plugin
reinstall turns the whole restore into an error even though file
restoration succeeded, and omitting `options` entirely still means
restore everything — that is the documented default, and it is only the
mis-serialized case that was wrong.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NohXi78cwsAKtN1sCfxjUh

web_interface/blueprints/api_v3.py

@@ -8181,7 +8181,16 @@ def backup_restore():
         try:
             opts_dict = json.loads(options_raw)
         except json.JSONDecodeError:
-            opts_dict = {}
+            opts_dict = None
+        if not isinstance(opts_dict, dict):
+            # Every option defaults to True, so falling back to {} on a
+            # parse failure would silently perform a FULL restore —
+            # secrets and all — for a caller who asked for a narrow one
+            # and mis-serialized it. Refuse instead of guessing.
+            return jsonify({
+                'status': 'error',
+                'message': 'Invalid options: expected a JSON object',
+            }), 400
         options = RestoreOptions(
             restore_config=bool(opts_dict.get('restore_config', True)),
             restore_secrets=bool(opts_dict.get('restore_secrets', True)),