Backend: edition image d'entete + logo des adherents

- Champs upload image d'entete (coverUrl) et logo (logoUrl) dans le
  formulaire d'edition d'un adherent (composant ImageField, fichier ->
  data-URL via hidden input)
- updateMember persiste desormais coverUrl + logoUrl, et revalide la
  fiche publique /adherents/[id]
- serverActions.bodySizeLimit a 4mb (les data-URL depassent la limite
  par defaut de 1mb)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/backend/actions.ts

@@ -141,6 +141,8 @@ export async function updateMember(formData: FormData) {
       phone: String(formData.get("phone") ?? "").trim() || null,
       website: String(formData.get("website") ?? "").trim() || null,
       memberSince: formData.get("memberSince") ? Number(formData.get("memberSince")) : null,
+      coverUrl: String(formData.get("coverUrl") ?? "").trim() || null,
+      logoUrl: String(formData.get("logoUrl") ?? "").trim() || null,
       tags: String(formData.get("tags") ?? "").trim() || null,
       hours: String(formData.get("hours") ?? "").trim() || null,
       status: String(formData.get("status") ?? "pending") as "active" | "pending",
@@ -149,6 +151,7 @@ export async function updateMember(formData: FormData) {
     .where(eq(members.id, id));
 
   revalidatePath("/backend/adherents");
+  revalidatePath(`/adherents/${id}`);
   revalidatePath("/");
 }