- Add `start_sync_task()` in agent/sync.rs: spawns a background task
that syncs immediately on app startup, then every 10 minutes.
- `sync_once()` performs a single pass; errors are logged and swallowed
so the periodic task never stops.
- Remove authentication requirement from OpenRouter API (no API key needed).
- Call `service.start_sync_task()` from main.rs after AppService init.
- Also update the existing `sync_upstream_models` (HTTP API) to remove
the now-unnecessary API key requirement for consistency.
