Add Gitea Actions CI workflow

Runs on push/PR to codex/staging-baseline plus manual dispatch: sbt test for
the backend, and npm ci/typecheck/build/audit (full + production-only) for
the frontend, matching the checks already documented as the manual Rocky
verification routine. Verified sbt test compiles and passes with no .env
file present (matching what a checkout-only CI job will actually have) via
a git-archive dry run before writing this.

Two separate jobs, each with its own container image, rather than one
shared runner-label image with ad-hoc installs — reuses the exact
hseeberger/scala-sbt image already used all session for local backend
builds, and a plain node:20-bookworm for the frontend, so nothing needs
apt-get bootstrapping of a second language runtime into the same container.

.gitea/workflows/ci.yml

@@ -0,0 +1,40 @@
+name: CI
+
+on:
+  push:
+    branches:
+      - codex/staging-baseline
+  pull_request:
+    branches:
+      - codex/staging-baseline
+  workflow_dispatch:
+
+jobs:
+  backend:
+    runs-on: docker
+    container:
+      image: hseeberger/scala-sbt:17.0.2_1.6.2_2.13.8
+    steps:
+      - uses: actions/checkout@v4
+      - name: sbt test
+        run: sbt -batch test
+
+  frontend:
+    runs-on: docker
+    container:
+      image: node:20-bookworm
+    defaults:
+      run:
+        working-directory: web
+    steps:
+      - uses: actions/checkout@v4
+      - name: npm ci
+        run: npm ci
+      - name: typecheck
+        run: npm run typecheck
+      - name: build
+        run: npm run build
+      - name: audit (full)
+        run: npm audit
+      - name: audit (production only)
+        run: npm audit --omit=dev