fix(security): decode settings at the API boundary and honour the transaction

Round-three review follow-ups.

getImageSecurityDefaults now accepts a transaction, the way getAppSetting
two lines above it already does. quoteService.convertToEvent called it
from inside db.transaction() through the global db; sqlite3 runs a
single-connection pool, so that read would have waited on the connection
its own transaction was holding until the acquire timeout, and the
helper's catch would then have swallowed the error and dropped the
defaults silently.

The double-encoding is fixed where it starts. GET
/admin/image-security/settings returned setting_value undecoded, so it
shipped "true" to a tab that types the field as boolean — and since the
tab PUTs the whole object back through JSON.stringify, every save
wrapped another layer around values nobody edited. It decodes now, so a
round trip is idempotent. The tab is the only consumer of that endpoint.

The reader unwraps to any depth instead of four. The depth on an
existing install is however many times someone opened that tab, which is
not a number to cap. It terminates because each parse of a string is
strictly shorter than its input.

Refs #1296

backend/__tests__/integration/imageSecurityDefaults.test.js

@@ -134,6 +134,15 @@ describe('image-security creation defaults', () => {
       expect(await getImageSecurityDefaults()).toEqual({ image_quality: 72 });
     });
 
+    it('reads a value buried under many saves, not just one', async () => {
+      // Each visit to the settings tab used to add a layer, so the depth is
+      // however many times someone opened it — not a number to cap.
+      let raw = JSON.stringify('maximum');
+      for (let i = 0; i < 8; i += 1) raw = JSON.stringify(raw);
+      await setRaw('default_protection_level', raw);
+      expect(await getImageSecurityDefaults()).toEqual({ protection_level: 'maximum' });
+    });
+
     it('still rejects a malformed value however many times it was encoded', async () => {
       await setRaw('default_image_quality', JSON.stringify(JSON.stringify('72oops')));
       expect(await getImageSecurityDefaults()).toEqual({});