refactor: convert to bun workspaces monorepo

- Move web app into apps/web/
- Create packages/shared/ with shared types
- Create plugins/decky-vault/ scaffold
- Root package.json manages workspaces only

apps/web/lib/api/health.ts

@@ -0,0 +1,19 @@
+import { Elysia } from "elysia"
+
+export const healthRoutes = new Elysia({
+  prefix: "/health",
+  detail: { tags: ["Health"] },
+}).get(
+  "/",
+  () => ({
+    status: "ok",
+    timestamp: new Date().toISOString(),
+    service: "deckyvault-api",
+  }),
+  {
+    detail: {
+      summary: "Health check",
+      description: "Returns the current health status of the API.",
+    },
+  },
+)