fix(server-storage): merge user data on move, keep failed writes, sign-in route gate

- A merged playlist gains the browser document's favorites and history;
  Xtream values for one server entry are combined (lists unioned, maps
  merged, the server winning conflicts) and written once.
- A failed user-data write stays queued and survives a re-read; the
  page-hide flush waits for a batch still travelling.
- The start-up screen lets the sign-in route through whatever its check
  reports.
- The backend refuses IPTVNATOR_ACCESS_TOKEN without server storage.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docker/README.md

@@ -103,7 +103,7 @@ safe local defaults shown below.
 | `NODE_EXTRA_CA_CERTS`                    | unset                   | Optional Node.js CA bundle path for providers using private certificate authorities. Mount the CA file into the container and set this path. |
 | `IPTVNATOR_STORAGE`                      | `server` in the image   | `server` opens the server storage database under `IPTVNATOR_DATA_DIR` and its `/api/v1` routes; `none` keeps the backend stateless.          |
 | `IPTVNATOR_DATA_DIR`                     | `/data` in the image    | Data root; the database is `<root>/databases/iptvnator.db`. Mount a volume here.                                                             |
-| `IPTVNATOR_ACCESS_TOKEN`                 | unset                   | When set, every route except `/`, `/health`, `/config.js` and `/v1/instance` requires `Authorization: Bearer <token>`, proxy routes included. Each browser asks for the token once and keeps it. |
+| `IPTVNATOR_ACCESS_TOKEN`                 | unset                   | When set, every route except `/`, `/health`, `/config.js` and `/v1/instance` requires `Authorization: Bearer <token>`, proxy routes included. Each browser asks for the token once and keeps it. Needs `IPTVNATOR_STORAGE=server`; the backend refuses to start otherwise. |
 | `IPTVNATOR_MAX_DOCUMENT_MB`              | `64`                    | Largest playlist document the backend accepts. Keep the nginx `client_max_body_size` in step.                                                |
 | `IPTVNATOR_BACKEND_START_TIMEOUT`        | `300`                   | Seconds the entrypoint waits for the backend's `/health` before giving up. Database migrations run before the backend listens.              |