feat(portal): show each season's own poster beside the season tabs (#1628)

Series detail pages now render the selected season's poster as a season
cover next to the season tabs and description, and the fullscreen episode
panel shows the same poster as a season strip above its tabs.

Resolution is TMDB-first, like the show artwork merge: the lazy season
enrichment stores `/tv/{id}/season/{n}` `poster_path` as a w342 URL in
`tmdb_season_posters` (Xtream) or `StalkerSeriesTmdbSeasonsService.posters()`
(Stalker), under the same write-only-if-changed convergence guard as the
season overview. Xtream falls back to the provider's `seasons[].cover_big`/
`cover` when it is an http(s) URL other than the show poster, because panels
repeat the show poster on every season. Stalker is TMDB-only.

The cover column is not rendered for one-season items, seasons without a
poster, or a failed image, so every fallback is today's markup. It is sized
by a new `--season-cover-width` token (96/120/144px per Settings.coverSize).
The hero poster never follows the season.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/architecture/embedded-inline-playback.md

@@ -278,6 +278,23 @@ episode" chip appears when the playing episode is outside the opened
 season. Season descriptions come from `get_series_info` seasons (Xtream)
 or `TmdbEnrichmentService.getSeason` (Stalker).
 
+The tabs and description sit in a **season card** with an optional cover
+column: the selected season's own poster (`seasonPosters` input, keyed like
+`seasonDescriptions`; TMDB season poster first, provider `seasons[].cover_big`
+next — see "Season/Episode Enrichment" in `tmdb-metadata-enrichment.md`).
+The column is sized by `--season-cover-width` (96 / 120 / 144px for
+`Settings.coverSize` small / medium / large, `apps/web/src/_cover-size.scss`;
+medium matches the About block's poster) and is not rendered at all — the
+card collapses to one column and the tabs render exactly as before — when
+the selected season has no poster, when the item has a single season (that
+poster is the show poster a few hundred pixels below the hero), or when the
+image request failed. The hero poster never follows the season: the show
+keeps its identity element, the season gets its own picture next to its own
+text. The fullscreen episode panel shows the same poster as a compact season
+strip (poster, season name, episode count) above its season tabs, under the
+same gates, from `PortalInlinePlayerComponent.seasonPosters` through
+`buildFullscreenEpisodePanelSeasons`.
+
 ### Manual watched toggle for movies
 
 Movies carry the same manual "watched" affordance as episodes, in the