feat: developer access permission gates webhooks/API keys/BYOK (v0.71.0)

Webhooks, self-serve API keys, and BYOK AI provider keys had zero
access gating -- any logged-in user, any tier. Adds users.isDeveloper
(boolean, admin-toggled in admin/users/[id] alongside role/tier),
checked via a single hasDeveloperAccess() (lib/permissions.ts) so a
future subscription-tier auto-grant is a one-line change there, not
a redesign across call sites.

requireDeveloper() (lib/api-auth.ts) wraps requireSession() with a
fresh isDeveloper check (same reasoning as requireAdmin re-querying
role: session.user's cookieCache can be up to 5 minutes stale) and
replaces requireSession in all 8 gated routes: webhooks CRUD +
deliveries + redeliver, api-keys CRUD, ai-keys CRUD.

Settings UI: the sidebar hides API Keys/Webhooks nav entries for
non-developers; those pages and the BYOK section of Settings -> AI
show a locked notice instead of the manager component when accessed
directly.

Migration grandfathers in anyone who already has a webhook, API key,
or BYOK key row -- ships as a new gate on existing features, not a
silent lockout of active integrations.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/app/admin/users/[id]/page.tsx

@@ -102,7 +102,7 @@ export default async function AdminUserDetailPage({ params }: PageProps) {
             <CardTitle className="text-base">Edit Role &amp; Tier</CardTitle>
           </CardHeader>
           <CardContent>
-            <UserEditor userId={user.id} currentRole={user.role} currentTier={user.tier} />
+            <UserEditor userId={user.id} currentRole={user.role} currentTier={user.tier} currentIsDeveloper={user.isDeveloper} />
           </CardContent>
         </Card>