feat: ProductMember — team management for product backlogs

- Add ProductMember model (many-to-many User ↔ Product)
- Add productAccessFilter helper (owner OR member OR clause)
- Replace all ownership checks across actions and API routes
- Add addProductMemberAction / removeProductMemberAction / leaveProductAction
- Add TeamManager component in product settings (owner adds/removes Developers)
- Add LeaveProductButton in user settings (member leaves a product team)
- Regenerate Prisma Client after schema migration

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

components/settings/leave-product-button.tsx

@@ -0,0 +1,44 @@
+'use client'
+
+import { useState, useTransition } from 'react'
+import { Button } from '@/components/ui/button'
+import { leaveProductAction } from '@/actions/products'
+
+interface LeaveProductButtonProps {
+  productId: string
+}
+
+export function LeaveProductButton({ productId }: LeaveProductButtonProps) {
+  const [confirming, setConfirming] = useState(false)
+  const [isPending, startTransition] = useTransition()
+
+  function handleLeave() {
+    startTransition(async () => {
+      await leaveProductAction(productId)
+    })
+  }
+
+  if (confirming) {
+    return (
+      <div className="flex gap-2 shrink-0">
+        <Button variant="destructive" size="sm" disabled={isPending} onClick={handleLeave}>
+          {isPending ? 'Bezig…' : 'Ja, verlaten'}
+        </Button>
+        <Button variant="ghost" size="sm" onClick={() => setConfirming(false)} disabled={isPending}>
+          Annuleren
+        </Button>
+      </div>
+    )
+  }
+
+  return (
+    <Button
+      variant="outline"
+      size="sm"
+      className="shrink-0 border-error/40 text-error hover:bg-error/10"
+      onClick={() => setConfirming(true)}
+    >
+      Verlaten
+    </Button>
+  )
+}