Add Forgejo Actions CI workflow

Type-check and build frontend + backend on push to main and on PRs.
Runner is registered on a dedicated LXC (forgejo-runner) using
node:22-bookworm containers.

Co-authored-by: Samuel James <ssamjame@amazon.com>
Co-authored-by: Kiro <noreply@kiro.dev>

.forgejo/workflows/ci.yml

@@ -0,0 +1,32 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: 22
+
+      - name: Install + type-check + build frontend
+        run: |
+          npm ci
+          npx tsc --noEmit
+          npm run build
+
+      - name: Install + type-check + build backend
+        working-directory: backend
+        run: |
+          npm ci
+          npx tsc --noEmit
+          npm run build