fix(webapp): drop unused OrgMember _count aggregate from org-list presenter (#4587)

## What

`OrganizationsPresenter.#getOrganizations` selected a Prisma
`_count.members` relation on every org-list load (hit on nearly every
dashboard navigation). Prisma lowers that relation `_count` to a
whole-`OrgMember`-table `GROUP BY organizationId` aggregate joined onto
`Organization`. The computed `membersCount` field is read by **nothing**
in the webapp, so the entire aggregate scan is wasted work.

This removes the `_count` select and the `membersCount` field. The query
keeps only the indexed `EXISTS` membership filter and the org/project
selects.

## Why it's safe

- `membersCount` has zero consumers (whole-webapp grep finds the name
only at the point of assignment). It was added in #1796 (2023) and has
been unused since.
- The member count shown on the org settings/team page comes from a
separate presenter query, not this one. No user-visible change.

## Evidence (generated SQL, before/after, seeded isolated stack)

Before (with `_count.members`):

```sql
SELECT ..., COALESCE(aggr._aggr_count_members, 0)
FROM "Organization"
LEFT JOIN (SELECT "organizationId", COUNT(*) AS _aggr_count_members
           FROM "OrgMember" GROUP BY "organizationId") aggr ON ...
WHERE EXISTS (... "userId" = $1 ...) AND "deletedAt" IS NULL
ORDER BY "createdAt" DESC
```

After:

```sql
SELECT id, slug, title, avatar, "featureFlags"
FROM "Organization"
WHERE EXISTS (... "userId" = $1 ...) AND "deletedAt" IS NULL
ORDER BY "createdAt" DESC
```

The whole-table `GROUP BY` aggregate is gone. The only remaining
`OrgMember` access is the `EXISTS` on the caller's own membership
(indexed by `userId`, a handful of rows). This is the single largest
read-amplification query on the control-plane database (~1.39B rows
read/day, ~719s DB CPU/day per Insights); removing it takes that portion
to zero.

Webapp typecheck passes.

## Rollout

Straight deploy, zero blast radius. Rollback is a plain revert, no data
migration.

refs TRI-13170

.server-changes/org-list-member-count.md

@@ -0,0 +1,6 @@
+---
+area: webapp
+type: improvement
+---
+
+Reduced database load when loading the dashboard by removing an unused organization member count that was being calculated on every page navigation.

apps/webapp/app/presenters/OrganizationsPresenter.server.ts

@@ -152,11 +152,6 @@ export class OrganizationsPresenter {
           },
           orderBy: { name: "asc" },
         },
-        _count: {
-          select: {
-            members: true,
-          },
-        },
       },
     });
 
@@ -191,7 +186,6 @@ export class OrganizationsPresenter {
           updatedAt: project.updatedAt,
           externalRef: project.externalRef,
         })),
-        membersCount: org._count.members,
       };
     });
   }