Upgrade all outbound Discord surfaces (personal webhooks, bot DMs, server
channels, request events) from bare title/description embeds to rich ones:
poster thumbnail, overview teaser, TMDB/IMDb/TVDB links, rating and genre
fields, content-rating footer, and a clickable title URL.
Artwork respects the v1 privacy contract via a new admin poster mode
(notifications.discord.poster_mode): "provider" (default) only emits public
provider-CDN URLs, "server" additionally presigns locally cached posters
from this server's image storage, "off" drops images entirely. Builders
never derive artwork URLs themselves; the sender layer resolves PosterURL
through System.discordPosterURL.
To keep provider-CDN URLs derivable after image caching rewrites
poster_path to a local storage key, media_items gains poster_source_path,
captured during cacheItemImages, preserved across refreshes that keep the
cached poster, and cleared on explicit poster overrides.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
