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/products/team-manager.tsx

@@ -0,0 +1,75 @@
+'use client'
+
+import { useActionState, useState, useTransition } from 'react'
+import { Button } from '@/components/ui/button'
+import { Input } from '@/components/ui/input'
+import { addProductMemberAction, removeProductMemberAction } from '@/actions/products'
+
+interface Member {
+  id: string
+  username: string
+}
+
+interface TeamManagerProps {
+  productId: string
+  members: Member[]
+}
+
+export function TeamManager({ productId, members }: TeamManagerProps) {
+  const [state, formAction, isPending] = useActionState(addProductMemberAction, null)
+  const [removingId, setRemovingId] = useState<string | null>(null)
+  const [, startTransition] = useTransition()
+
+  function handleRemove(memberId: string) {
+    setRemovingId(memberId)
+    startTransition(async () => {
+      await removeProductMemberAction(productId, memberId)
+      setRemovingId(null)
+    })
+  }
+
+  return (
+    <div className="space-y-4">
+      {members.length === 0 ? (
+        <p className="text-sm text-muted-foreground">Nog geen teamleden toegevoegd.</p>
+      ) : (
+        <ul className="space-y-2">
+          {members.map(m => (
+            <li key={m.id} className="flex items-center justify-between gap-3 rounded-lg bg-surface-container px-3 py-2">
+              <span className="text-sm font-medium text-foreground">{m.username}</span>
+              <Button
+                variant="ghost"
+                size="sm"
+                className="text-error hover:bg-error/10 h-7 px-2 text-xs"
+                disabled={removingId === m.id}
+                onClick={() => handleRemove(m.id)}
+              >
+                {removingId === m.id ? 'Verwijderen…' : 'Verwijder'}
+              </Button>
+            </li>
+          ))}
+        </ul>
+      )}
+
+      <form action={formAction} className="flex gap-2">
+        <input type="hidden" name="productId" value={productId} />
+        <Input
+          name="username"
+          placeholder="Gebruikersnaam van Developer"
+          className="flex-1"
+          disabled={isPending}
+        />
+        <Button type="submit" size="sm" disabled={isPending}>
+          {isPending ? 'Toevoegen…' : 'Toevoegen'}
+        </Button>
+      </form>
+
+      {state && 'error' in state && typeof state.error === 'string' && (
+        <p className="text-xs text-error">{state.error}</p>
+      )}
+      {state && 'success' in state && (
+        <p className="text-xs text-success">Teamlid toegevoegd.</p>
+      )}
+    </div>
+  )
+}