Retry the remaining plex.tv calls that had no retry

[Brandon-Haney]

Two plex.tv entry points were still unretried while their siblings already had backoff.

The watchlist fetch

Constructing MyPlexAccount in the user is None branch of _fetch_user_watchlist was the last one without a retry. _get_main_account() and the switchHomeUser path both got one earlier. On an install with no home users that branch is the only watchlist fetch there is, so a single read timeout cost the whole run its array restore:

[USER:Brandon] Fetching watchlist media
ERROR (get Plex account for Brandon): plex.tv ... Read timed out
WARNING Skipping array restore - watchlist data incomplete

It now goes through _retry_plextv_call for the same 3 attempts with 2s/4s backoff as its siblings. Auth failures still fail on the first attempt, and exhausting the retries still guards array restore exactly as before.

The Settings user list

myPlexAccount() and users() both back /settings/plex/users and both are plex.tv round trips. A read timeout during the hourly refresh dropped the admin from the list until the next cycle, while the shared users loaded fine from the very next call:

Refreshing Plex data cache...
WARNING Could not get main account: plex.tv ... Read timed out
INFO Fetched 24 shared users

Capped at 2 attempts rather than PLEXTV_MAX_RETRIES, because this sits on a page request. Attempts here are paid in page latency, and giving up costs a stale user list rather than a skipped array restore.

How to test

1. On a single-user install (no home users), run a cache operation while plex.tv is briefly unreachable. The run retries instead of logging Skipping array restore - watchlist data incomplete on the first timeout.
2. Open Settings then Plex then Users during a plex.tv blip. The admin account stays in the list rather than disappearing until the next refresh.
3. Confirm a genuine auth failure (bad token) still fails immediately rather than retrying.

Covered by tests/test_plex_api_main_account_retry.py and tests/test_settings_service_plextv_retry.py.