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/api/admin/plugins/[key]/route.ts

@@ -0,0 +1,39 @@
+import { NextResponse } from "next/server";
+import { z } from "zod";
+import { requireRole } from "@/lib/authorization";
+import { UserRole } from "@/generated/prisma/enums";
+import { findDescriptor } from "@/lib/plugins/registry";
+import { togglePlugin, updatePluginConfig, getPluginState } from "@/lib/plugins/server";
+
+const patchSchema = z.object({
+  enabled: z.boolean().optional(),
+  config: z.record(z.string(), z.unknown()).optional(),
+});
+
+export async function PATCH(req: Request, ctx: { params: Promise<{ key: string }> }) {
+  await requireRole([UserRole.ADMIN]);
+  const { key } = await ctx.params;
+  if (!findDescriptor(key)) {
+    return NextResponse.json({ error: "Unknown plugin" }, { status: 404 });
+  }
+  const parsed = patchSchema.safeParse(await req.json().catch(() => ({})));
+  if (!parsed.success) {
+    return NextResponse.json({ error: "Invalid payload" }, { status: 400 });
+  }
+  let state = await getPluginState(key);
+  if (parsed.data.enabled !== undefined) {
+    state = await togglePlugin(key, parsed.data.enabled);
+  }
+  if (parsed.data.config !== undefined) {
+    state = await updatePluginConfig(key, parsed.data.config);
+  }
+  return NextResponse.json(state);
+}
+
+export async function GET(_req: Request, ctx: { params: Promise<{ key: string }> }) {
+  await requireRole([UserRole.ADMIN]);
+  const { key } = await ctx.params;
+  const state = await getPluginState(key);
+  if (!state) return NextResponse.json({ error: "Not found" }, { status: 404 });
+  return NextResponse.json(state);
+}