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, 32 insertions, 1 deletions

diff --git a/packages/meshbay-node/src/meshbay_node/indexer/group_index.py b/packages/meshbay-node/src/meshbay_node/indexer/group_index.py
index ec98667..1ce4e0a 100644
--- a/packages/meshbay-node/src/meshbay_node/indexer/group_index.py
+++ b/packages/meshbay-node/src/meshbay_node/indexer/group_index.py
@@ -78,6 +78,19 @@ class GroupIndex:
     def get_entry(self, file_id: str) -> IndexEntry | None:
         return self._entries.get(file_id)
 
+    def get_entry_by_path(self, path: str) -> IndexEntry | None:
+        """
+        Linear scan — entries are keyed by content id, not path, and nothing
+        before the Videos app needed to go the other way (a client always
+        already has the id from index_sync/index_delta). Fine for an
+        on-demand, per-tile lookup against a few thousand entries; revisit
+        if a future caller makes this hot.
+        """
+        for entry in self._entries.values():
+            if entry.path == path:
+                return entry
+        return None
+
     @property
     def entries(self) -> list[IndexEntry]:
         return list(self._entries.values())
@@ -205,16 +218,34 @@ class GroupIndex:
     # ── Delta ─────────────────────────────────────────────────────────────────
 
     def diff(self, previous: "GroupIndex") -> IndexDelta:
-        """Compute what changed since a previous version of this index."""
+        """
+        Compute what changed since a previous version of this index.
+
+        A shared id whose entry object now compares unequal (field-by-field,
+        via IndexEntry's dataclass-generated __eq__) is an update, not an
+        addition — the Videos app's async enrichment (duration, thumb_hash,
+        title, ...) replaces an existing entry's fields after the fact via
+        `add_entry`, which never introduces a new id. This only works
+        because that replacement always constructs a *new* IndexEntry object
+        (`dataclasses.replace`, never in-place attribute mutation) — mutating
+        the same object in place would also mutate `previous`'s copy, since
+        entries_by_id() is a shallow dict copy, and the two would always
+        compare equal.
+        """
         prev_ids = set(previous._entries)
         curr_ids = set(self._entries)
 
         additions = [self._entries[i] for i in curr_ids - prev_ids]
         deletions = list(prev_ids - curr_ids)
+        updates = [
+            self._entries[i] for i in curr_ids & prev_ids
+            if self._entries[i] != previous._entries[i]
+        ]
 
         return IndexDelta(
             base_version=previous.version,
             version=self.version,
             additions=additions,
             deletions=deletions,
+            updates=updates,
         )