feat(node,hub): add Videos group app (poster grid, flat list, TMDB metadata)

Implements docs/mediacenter.md: a "Videos" group application built on the
existing files index rather than a separate catalogue. On the node side,
new indexer enrichment (technical probe, filename/season parsing, thumbnail
generation) runs per-file once an operator has chosen a video_root for the
group, plus a TMDB client for on-demand poster/metadata lookups (never
client-side, thumbnails delivered over the existing chunk path). On the hub
side, a new video-app.js renders a lazily-mounted poster grid or a
thumbnail-only flat list, with TMDB entirely optional per group.

Along the way: the global apps registry now drives Settings' default-tab
picker instead of a hardcoded list, and the video_root is configured from
group Settings (like uploads) rather than from Files, with the node
refusing to run any TMDB/thumbnail work until one is set.

Fixes several bugs found via live testing against a real library, notably
a race between two effects writing the same "image ready" state that could
leave a poster grid spinning forever on a same-tab revisit — see
mediacenter.md §5.4 for the full account of each one.

1 files changed, 2 insertions, 0 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/static/apps.js b/packages/meshbay-hub/src/meshbay_hub/static/apps.js
index 0db713c..88b4a0f 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/apps.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/apps.js
@@ -1,5 +1,6 @@
 import { ChatPanel } from './chat-app.js';
 import { FilesPanel } from './files-app.js';
+import { VideoApp } from './video-app.js';
 
 /**
  * Every group "application", in tab order.
@@ -16,6 +17,7 @@ import { FilesPanel } from './files-app.js';
 const APPS = [
   { key: 'chat', icon: 'chat', labelKey: 'group.tab_chat', Component: ChatPanel },
   { key: 'files', icon: 'folder', labelKey: 'group.tab_files', Component: FilesPanel },
+  { key: 'video', icon: 'video', labelKey: 'group.tab_video', Component: VideoApp },
 ];
 
 /** The registry filtered to what this group has enabled, in registry order. */