Key sync runs inside the app: scheduler addon task every 10 min, plus write guards

The per-member key seed ran from a steward cron on the co-op agent VM, shelling
into this app over the Cloudron API. That made a member-facing function depend on
a separate machine's uptime and paths (it went silently stale once, exiting 127
on schedule while the job looked healthy).

- CloudronManifest.json: addons.scheduler gains librechat_key_sync
  (cd /app/code/librechat/api && node /app/data/sync-keys.js, */10 * * * *).
  Official addon set (postgresql, mongodb, localstorage, oidc, sendmail) and the
  dockerImage pin are untouched.
- sync-keys.js: refuse to write unless CREDS_KEY can first decrypt an existing
  key row, and unless each fresh ciphertext round-trips back to the plaintext.
  Scheduled runs get a different environment than an interactive exec, so a wrong
  or absent CREDS_KEY must fail loudly instead of overwriting working member keys.

CloudronManifest.json

@@ -22,6 +22,12 @@
         },
         "sendmail": {
             "supportsDisplayName": true
+        },
+        "scheduler": {
+            "librechat_key_sync": {
+                "schedule": "*/10 * * * *",
+                "command": "cd /app/code/librechat/api && node /app/data/sync-keys.js"
+            }
         }
     },
     "checklist": {