Tags collection uses side effect in select function to write uncategorized counts

[brendanlong]

This is related to #580 and a PR fixing this should target the tanstack-db branch.

Problem

In src/lib/collections/tags.ts:60-76, the tags collection's select function writes uncategorized counts to the counts collection as a side effect:

select: (data: TagsListResponse) => {
  // Side effect: write uncategorized counts to the counts collection
  const existing = countsCollection.get("uncategorized");
  if (existing) {
    countsCollection.update("uncategorized", (draft) => {
      draft.total = data.uncategorized.feedCount;
      draft.unread = data.uncategorized.unreadCount;
    });
  } else {
    countsCollection.insert({
      id: "uncategorized",
      total: data.uncategorized.feedCount,
      unread: data.uncategorized.unreadCount,
    });
  }

  return data.items;
},

Why this is fragile

1. select can be called multiple times by TanStack Query (e.g., on re-renders, cache reads). While the logic is idempotent (insert-or-update), side effects in select violate the expected contract that select is a pure transformation.

2. Implicit ordering dependency: The counts collection must be created before the tags collection. This is currently handled correctly in createCollections() with a comment, but it's a hidden invariant.

3. Mixing concerns: The tags collection is responsible for its own data and for populating a different collection's data. This makes the data flow harder to trace.

Suggestion

Move the uncategorized count update out of select and into a more explicit location. Some options:

• Wrap the tags fetch so it returns items and writes uncategorized counts as two separate steps (e.g., a callback after the queryFn resolves)
• Use a separate lightweight query for uncategorized counts (could be part of the existing entries.count prefetch)
• Use queryClient.getQueryCache().subscribe() to watch for tags.list updates and write uncategorized counts in response
• Split the API response — have tags.list return only tags, and uncategorized counts come from a different endpoint that seeds the counts collection directly