Add CI/CD: Gitea Actions workflows + Act Runner

- .gitea/workflows/test.yml: unit tests + build on every push
- .gitea/workflows/deploy.yml: auto deploy to production on push to main
- .gitea/workflows/preview.yml: PR preview environments at pr-{id}.oil.oci.euphon.net
  - Bakes production DB copy into preview image (no PVC needed)
  - Auto-creates namespace + deployment + ingress with TLS
  - Comments PR with preview URL
  - Tears down on PR close
- scripts/setup-runner.sh: act_runner installation script

Runner: hera-runner (host mode, ubuntu-latest label)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/test.yml

@@ -0,0 +1,17 @@
+name: Test
+on: [push]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install dependencies
+        run: cd frontend && npm ci
+
+      - name: Unit tests
+        run: cd frontend && npm run test:unit
+
+      - name: Build check
+        run: cd frontend && npm run build