ci: make typecheck a real blocking gate; bump actions to Node 24 majors

The previous `vue-tsc --noEmit` step was a no-op: the root tsconfig uses
`files: []` with project references, so without `--build` it checks zero files
and always passes. Switch to `vue-tsc --build` (new `yarn typecheck` script)
and make the job blocking.

Fix the one error this surfaced: UsageEventDocument extended mongoose.Document,
whose `model` method collides with the `model: string` field. Use the
recommended pattern — a plain attrs interface passed to the Schema/Model
generics (hydrated docs still expose Document methods). Typecheck is now clean.

Bump actions/checkout@v5 and actions/setup-node@v5 to silence the Node.js 20
runtime deprecation (forced to Node 24 from 2026-06-16).

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

package.json

@@ -11,6 +11,7 @@
     "preview": "nuxt preview",
     "start": "node .output/server/index.mjs",
     "postinstall": "nuxt prepare",
+    "typecheck": "vue-tsc --build",
     "sharp:rebuild": "SHARP_IGNORE_GLOBAL_LIBVIPS=1 yarn rebuild sharp",
     "import:decision": "tsx --tsconfig tsconfig.scripts.json scripts/import-decision-tree.ts",
     "test": "tsx --tsconfig tsconfig.tests.json --test \"tests/**/*.test.ts\" \"server/**/*.test.ts\" \"shared/**/*.test.ts\""