feat(vps): port admin to HttpApiBuilder.group (Step 6)

Ports GET /api/admin/overview, /api/admin/frontend-errors/:scenario, and
GET /api/admin/newsletter-subscribers from the Hono/zod-openapi router
onto the Effect router. All three are admin-gated via .middleware(AuthMiddleware)
plus an in-handler role check (requireAdmin), matching the pattern already
used for music-artists' write endpoints.

effect@4.0.0-beta.93's HttpApiError has no built-in 429 -- the frontend
error simulator (a QA tool exercising www's error boundary/Sentry-reporting
threshold against every real status code) needs one, so
SimulatedRateLimitError is declared in packages/api/src/admin.ts the same
way Phase 2 of the migration doc ports any other domain error. The other
simulated-error branches use the built-in HttpApiError classes directly
(bodyless) rather than preserving the old handler's { error, scenario }
response body -- confirmed via apps/www/src/lib/http-client.ts that the
frontend only branches on res.status, not response body shape.

getAdminOverview's ~30-query aggregation batch has no dedicated service
layer (same as the old Hono handler); ported as one Effect.tryPromise
wrapping the whole batch, matching the old handler's single try/catch
around one Promise.all rather than converting each query individually.

Deletes the superseded routes/admin/{admin.routes,admin.index,admin.handlers}.ts
Hono trio and its app.ts mount. No apps/www changes -- fetcher() calls the
same /api/admin/* paths unchanged, verified end-to-end against the real
prod-connected dev server (admin overview dashboard + newsletter tab both
render real data through the new handler, logged in via a local dev
account).

Author: guidefari

apps/vps/src/app.ts

@@ -1,7 +1,6 @@
 import { Duration, Effect, Schedule } from 'effect'
 import configureOpenAPI from '@/lib/configure-open-api'
 import { createAppEffect } from '@/lib/create-app'
-import admin from '@/routes/admin/admin.index'
 import content from '@/routes/content/content.index'
 import email from '@/routes/email/email.index'
 import favorites from '@/routes/favorites/favorites.index'
@@ -29,7 +28,6 @@ const setupRoutesEffect = Effect.gen(function* () {
 
   configureOpenAPI(app)
 
-  app.route('/api/admin', admin)
   app.route('/api/favorites', favorites)
   app.route('/api/invite', invite)
   app.route('/api/resolve', resolve)