feat(plugins): foundation système Plugin Karbé

- Modèle Prisma Plugin (key, name, description, category, version, enabled,
  config JSONB, migrationsApplied, timestamps) + migration SQL
- PluginRegistry (src/lib/plugins/registry.ts) avec 12 plugins déclarés :
  visuels (theme-guyane, landing-hero, landing-sections, image-gallery-seed,
  demo-carbets-seed), métier (access-type, seasonality, pirogue-providers,
  min-stay), contenus (content-pages, legal-pages), i18n (i18n-fr-en)
- Server helpers (server.ts) : sync, isEnabled, getEnabledKeys, toggle avec
  hooks onEnable/onDisable, updateConfig, cache 5s
- Client bridge (client.tsx) : PluginProvider + useIsPluginEnabled
- Composant <IfPluginEnabled plugin=... fallback=...>
- Guard requirePluginOr404 pour pages et routes
- Page admin /admin/plugins avec table toggle par catégorie + édition config
- Route PATCH /api/admin/plugins/[key] + GET
- Layout async qui sync registry + passe enabledKeys au PluginProvider

Tous plugins en enabled=false par défaut, activation pilotée depuis l'admin.

src/app/admin/plugins/_components/PluginToggleTable.tsx

@@ -0,0 +1,102 @@
+"use client";
+
+import { useState, useTransition } from "react";
+
+interface PluginRow {
+  key: string;
+  name: string;
+  description: string;
+  category: string;
+  version: string;
+  enabled: boolean;
+  config: Record<string, unknown>;
+}
+
+const CATEGORY_LABEL: Record<string, string> = {
+  visual: "Visuels",
+  business: "Métier",
+  content: "Contenus",
+  i18n: "Internationalisation",
+  core: "Core",
+};
+
+export default function PluginToggleTable({ plugins: initial }: { plugins: PluginRow[] }) {
+  const [plugins, setPlugins] = useState(initial);
+  const [pending, startTransition] = useTransition();
+  const [busyKey, setBusyKey] = useState<string | null>(null);
+  const [error, setError] = useState<string | null>(null);
+
+  const byCategory = plugins.reduce<Record<string, PluginRow[]>>((acc, p) => {
+    (acc[p.category] ??= []).push(p);
+    return acc;
+  }, {});
+
+  async function toggle(key: string, next: boolean) {
+    setError(null);
+    setBusyKey(key);
+    try {
+      const res = await fetch(`/api/admin/plugins/${encodeURIComponent(key)}`, {
+        method: "PATCH",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ enabled: next }),
+      });
+      if (!res.ok) {
+        const body = await res.json().catch(() => ({}));
+        throw new Error(body?.error || `HTTP ${res.status}`);
+      }
+      const updated = await res.json();
+      startTransition(() => {
+        setPlugins((curr) =>
+          curr.map((p) => (p.key === key ? { ...p, enabled: !!updated.enabled } : p)),
+        );
+      });
+    } catch (e) {
+      setError(e instanceof Error ? e.message : String(e));
+    } finally {
+      setBusyKey(null);
+    }
+  }
+
+  return (
+    <div className="space-y-8">
+      {error && (
+        <div className="rounded-md border border-red-300 bg-red-50 px-3 py-2 text-sm text-red-700">
+          {error}
+        </div>
+      )}
+      {Object.entries(byCategory).map(([category, rows]) => (
+        <section key={category}>
+          <h2 className="mb-2 text-sm font-semibold uppercase tracking-wide text-gray-500">
+            {CATEGORY_LABEL[category] ?? category}
+          </h2>
+          <ul className="divide-y divide-gray-200 rounded-lg border border-gray-200 bg-white">
+            {rows.map((p) => (
+              <li key={p.key} className="flex items-start justify-between gap-4 px-4 py-3">
+                <div className="min-w-0">
+                  <div className="flex items-center gap-2">
+                    <span className="font-medium text-gray-900">{p.name}</span>
+                    <code className="rounded bg-gray-100 px-1.5 py-0.5 text-xs text-gray-600">{p.key}</code>
+                    <span className="text-xs text-gray-400">v{p.version}</span>
+                  </div>
+                  <p className="mt-1 text-sm text-gray-600">{p.description}</p>
+                </div>
+                <button
+                  type="button"
+                  onClick={() => toggle(p.key, !p.enabled)}
+                  disabled={pending || busyKey === p.key}
+                  className={`shrink-0 rounded-full px-3 py-1 text-xs font-semibold transition ${
+                    p.enabled
+                      ? "bg-green-600 text-white hover:bg-green-700"
+                      : "bg-gray-200 text-gray-700 hover:bg-gray-300"
+                  } disabled:opacity-50`}
+                >
+                  {busyKey === p.key ? "…" : p.enabled ? "Activé" : "Désactivé"}
+                </button>
+              </li>
+            ))}
+          </ul>
+        </section>
+      ))}
+    </div>
+  );
+}