ci: merge build/deploy jobs and cache stars by date

Collapse the two-job workflow into one, set the github-pages
environment on the single job, and key the star-data cache by
date (YYYY-MM-DD) so it is shared across same-day runs instead
of being per-run-id. Also skip tests on scheduled runs and
validate JSON before serving it.

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

.github/workflows/ci.yml

@@ -0,0 +1,33 @@
+name: CI
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+
+      - name: Install uv
+        uses: astral-sh/setup-uv@v7
+        with:
+          enable-cache: true
+
+      - name: Install dependencies
+        run: uv sync --group build
+
+      - name: Run tests
+        run: make test
+
+      - name: Build website
+        run: make build