fix(xtream): drop URL-only season overviews and fall back to TMDB (#1382)

Xtream panels routinely fill get_series_info seasons[].overview with a
bare cover-image URL, which rendered verbatim under the season tabs.
URL-only overviews are now treated as absent (sanitizeProviderOverview),
and the lazy season enrichment stores the TMDB season overview on the
selection (tmdb_season_overviews) as the fallback description - same
cached /tv/{id}/season/{n} payload, so no extra requests. Provider text
keeps priority when it is real prose.

The enrichment write is also convergent now: the serial detail re-fires
season enrichment after every selection write, and the previous
unconditional rewrite scheduled the next cache-served run indefinitely.
A repeat run that changes nothing no longer writes.

buildSeasonDescriptions is extracted from SerialDetailsComponent, which
would otherwise cross the 400-line max-lines limit.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

.changes/xtream-season-description-url.md

@@ -0,0 +1,9 @@
+---
+type: fix
+area: xtream
+---
+
+Series pages no longer show a raw image link under the season tabs when the
+provider fills the season description with a cover URL. URL-only descriptions
+are dropped, and with TMDB metadata enabled the missing season description is
+filled from TMDB instead.