Keep posters and casts between runs

Posters (the downloaded bytes, per size, 30 days, at most 200 MB) and
casts (people.json, 7 days, at most 2000) are stored in the user's cache
directory (os.UserCacheDir()/arr-tui) and used on the next start before
asking the servers. Expired posters are pruned at startup, casts are
written shortly after new ones arrive and on exit. Tests turn the cache
off.

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

SPECS.md

@@ -71,8 +71,9 @@ Notes:
 - **Download notifications:** a background poller (independent of the visible view) reads SABnzbd's history every 15 s and announces downloads that reach `Completed` or `Failed` as a popup and log entry; the first poll only records the current state. `ui.notify_downloads` (default true) and `ui.bell` (default false, rings the terminal bell) control it. Connection errors are not reported here, the indicators and the log already do that.
 - **Trakt lists:** while the search box of Movies or Series is empty, the list shows one of Trakt's public lists instead of search results: trending now (`/movies|shows/trending`, "N watching"), most watched this week (`/…/watched/weekly`, "N watched it"), anticipated (`/…/anticipated`, "on N lists"), popular (`/…/popular`), and for movies the US box office of last weekend (`/movies/boxoffice`, "$N last weekend"); 20 titles each (`page=1&limit=20`), with the headers `trakt-api-key` (the client ID, `trakt.client_id` or `ARR_TUI_TRAKT_CLIENT_ID`), `trakt-api-version: 2`, `Content-Type` and `User-Agent`. Every entry is looked up in Radarr (`tmdb:<id>`) or Sonarr (`tvdb:<id>`), at most four at a time, so it behaves like a search result (details, poster, tick when in the library, add, releases); entries without that id or unknown to the service are left out, in list order otherwise. `[` `]` switch lists (also from the empty search box), `r` reloads; a fetched list is shown again for 15 minutes without asking. Without a client ID the list says how to get one. Changing the client ID or a service in the Config view drops the fetched lists. `ARR_TUI_TRAKT_URL` points the client at another server (used by `dev/screenshots`).
 - **Downloading in the Library:** while the Library is visible it reads the Radarr and Sonarr queues (`/queue`, matched to titles by `movieId` / `seriesId`) when it opens, every 10 s, and 3 s after a grab. A title with queue entries gets a blue mark (filled when monitored, hollow when not) instead of the green or grey one, and its details a line "Downloading 45% · 0:06:12 left" (series: "Downloading N episodes"). A queue that cannot be read leaves the marks as they were. After a successful grab the release list closes (if nothing was opened on top of it).
+- **Disk cache:** posters and casts are kept between runs in `os.UserCacheDir()/arr-tui` (`$XDG_CACHE_HOME` or `~/.cache` on Linux, `~/Library/Caches` on macOS; directories `0700`, files written atomically). Posters are the downloaded bytes, one file per poster and size (small copy for block characters, large for real images) under `posters/<hash>.img`; a file is used for 30 days (by its modification time) and dropped when it cannot be decoded; at startup files older than that are deleted, then the oldest until the rest fit 200 MB. Casts are one `people.json` (key = `movie:tmdb:<id>` / `show:tvdb:<id>`, with the time it was fetched); entries are used for 7 days, at most 2000 are kept (the oldest go first), titles Trakt does not know are kept as having no cast; new casts are written 5 s after the last one and when the program quits. Changing the Trakt client ID keeps the casts and only forgets failed lookups.
 - **Season posters:** the series drill-down reads the series with `includeSeasonImages=true` (Sonarr only sends season posters when asked, and keeps no copy of them: they are TheTVDB links, fetched without credentials). A selected season or episode shows its season's poster; seasons without one, or whose poster cannot be loaded, show the series poster.
-- **Cast:** with a Trakt client ID, every details pane (search results, Trakt lists, Library, Calendar, the series drill-down) shows the director(s) of a movie or the "created by" credits of a series and up to 8 cast members with their characters below the plot ("and N more" after that). The title is found on Trakt by its TMDb id (movies) or TVDB id (series), else IMDb id (`/search/<type>/<id>?type=movie|show`), then `/movies|shows/<trakt id>/people` is read. Casts are cached for the session (up to 500 titles; a title Trakt does not know counts as having none; failures are retried after two minutes) and, like posters, only loaded once the selection has rested for 120 ms, so a held arrow key does not spend Trakt's allowance (500 requests per five minutes).
+- **Cast:** with a Trakt client ID, every details pane (search results, Trakt lists, Library, Calendar, the series drill-down) shows the director(s) of a movie or the "created by" credits of a series and up to 8 cast members with their characters below the plot ("and N more" after that). The title is found on Trakt by its TMDb id (movies) or TVDB id (series), else IMDb id (`/search/<type>/<id>?type=movie|show`), then `/movies|shows/<trakt id>/people` is read. Casts are cached, also on disk (see Disk cache; failures are retried after two minutes) and, like posters, only loaded once the selection has rested for 120 ms, so a held arrow key does not spend Trakt's allowance (500 requests per five minutes).
 - **Config view (F6):** one list with a field for every setting of the config file, one below the other under the headings Radarr, Sonarr, SABnzbd, Trakt and Interface (a single scrolling form; ↑↓ passes over the headings and ↓ after the last field reaches the Save / Revert buttons, so no Tab is needed; Tab jumps between the fields and the buttons). The `Test` button is part of the list, right below the SABnzbd settings; `Save` and `Revert` are centered in a bar below the list. When the config file does not exist, it is created empty (`config.Create`: directory `0700`, file `0600`) and the program opens on this view. A service with neither address nor key is "not configured": allowed by validation (half-filled ones are not), its client answers every call with `ErrNotConfigured` without a request, its indicator stays grey (logged once as information, not as unreachable) and its lists say "not configured (Config view, F6)" instead of an error. The fields start from the settings in use; API keys are masked (a checkbox shows them). The default quality profile and root folder of Radarr and Sonarr are dropdowns filled from the server (`/qualityprofile`, `/rootfolder`) with the address and key currently in the fields: when the view opens, on Revert, and 700 ms after the address or key was last changed; answers for values that have changed since are dropped, a failure is reported once in the result box. "(none)" comes first; a configured value the server does not list stays as an option, marked "(not on the server)", so opening the view never changes a setting. `Test` builds temporary clients from the typed values and reports each service. `Save` validates like startup does (URLs, non-empty keys, refresh 1–3600 s), then writes the file from the file's own contents plus the edits (`config.LoadFile`), so values that only come from `ARR_TUI_*` environment variables are left as the file has them and are shown greyed out; the write is atomic (temp file + rename) and keeps the file's permissions (`0600` when new); defaults such as `covers: true` stay out of the file unless it already named them. Everything then applies without a restart: the Radarr/Sonarr/SABnzbd clients are re-pointed (`Reconfigure`, guarded by a mutex), the connection indicators, add-form defaults and options are refreshed, the library and calendar reload, the Downloads interval, notifications, bell and poster style are switched live.
 - **Notifications and log (F7):** feedback about actions (added, deleted, grabbed, failures) appears as popups in the bottom right that fade away (errors stay longer). Everything is also kept in the Log view; only connection changes of the background polling are logged, not every failed poll.