docs: promote /web from develop — v1.2.3 changelog + Backups video + install page refresh + PBS keyfile docs polish

Consolidates the /web changes that were made on develop for the v1.2.3 stable
release, so the release PR (develop → main) can focus on version.txt +
AppImage + shell scripts without web conflicts.

Contents:

- CHANGELOG.md + web/data/changelog/es.md — v1.2.3 stable entry prepended
  (bullets + tables, not paragraphs) with the release header image.
- images/ProxMenux_backup.png — new v1.2.3 release header image.
- web/app/[locale]/docs/backup-restore/page.tsx +
  web/messages/{en,es}/docs/backup-restore/index.json — new Video walkthrough
  block for the YouTube demo of the Backups feature (videoId K7A1KtOe4IQ),
  embedded high on the overview page via the shared YouTubeEmbed component.
- web/app/[locale]/docs/installation/page.tsx +
  web/messages/{en,es}/docs/installation.json +
  web/public/install/install.png — stale Translation-flavour and Python-from-
  PyPI mentions removed from the install page; screenshot refreshed and moved
  to local /install/install.png (was pointing at the legacy GitHub Pages URL).
- web/messages/es/docs/backup-restore/destinations/pbs.json — minor polish
  aligned with PR #247 already on main.
- web/public/images/docs/backup-restore/pbs-paired-backup-groups.png — updated
  screenshot to match the reworked PBS-paired-group naming shipped in the beta
  cycle.
- images/logos_txt/jc_channel.txt — new asset for the contributors page.

web/app/[locale]/docs/backup-restore/page.tsx

@@ -4,6 +4,7 @@ import { Link } from "@/i18n/navigation"
 import { DocHeader } from "@/components/ui/doc-header"
 import { Callout } from "@/components/ui/callout"
 import { DataFlowDiagram } from "@/components/ui/data-flow-diagram"
+import { YouTubeEmbed } from "@/components/ui/youtube-embed"
 
 export async function generateMetadata({
   params,
@@ -81,6 +82,19 @@ export default async function BackupRestoreOverviewPage({
         {t.rich("intro.body", { code, strong })}
       </Callout>
 
+      <h2 className="text-2xl font-semibold mt-10 mb-4 text-gray-900">
+        {t("video.heading")}
+      </h2>
+      <p className="mb-4 text-gray-800 leading-relaxed">
+        {t("video.intro")}
+      </p>
+
+      <YouTubeEmbed
+        videoId="K7A1KtOe4IQ"
+        title={t("video.title")}
+        caption={t("video.caption")}
+      />
+
       <h2 className="text-2xl font-semibold mt-10 mb-4 text-gray-900">
         {t("whatItIsNot.heading")}
       </h2>

web/app/[locale]/docs/installation/page.tsx

@@ -97,7 +97,7 @@ export default async function InstallationPage({ params }: { params: Promise<{ l
       <p className="mb-4 text-gray-800 leading-relaxed">{t.rich("during.outro", { code, strong })}</p>
 
       <Image
-        src="https://macrimi.github.io/ProxMenux/install/install.png"
+        src="/install/install.png"
         alt={t("during.imageAlt")}
         width={900}
         height={500}