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>

scripts/setup-runner.sh

@@ -0,0 +1,56 @@
+#!/bin/bash
+# Setup Gitea Act Runner on this machine (host mode)
+# Usage: bash scripts/setup-runner.sh <registration-token>
+
+set -e
+
+TOKEN="${1:?Usage: $0 <registration-token>}"
+INSTANCE="https://git.euphon.cloud"
+RUNNER_NAME="hera-runner"
+RUNNER_BIN="$HOME/bin/act_runner"
+VERSION="v0.2.11"
+
+echo "=== Installing act_runner ${VERSION} ==="
+mkdir -p "$HOME/bin"
+curl -L "https://gitea.com/gitea/act_runner/releases/download/${VERSION}/act_runner-${VERSION}-linux-amd64" \
+  -o "$RUNNER_BIN"
+chmod +x "$RUNNER_BIN"
+echo "Installed: $($RUNNER_BIN --version)"
+
+echo ""
+echo "=== Registering runner ==="
+cd "$HOME"
+$RUNNER_BIN register --no-interactive \
+  --instance "$INSTANCE" \
+  --token "$TOKEN" \
+  --name "$RUNNER_NAME" \
+  --labels "ubuntu-latest:host"
+
+echo ""
+echo "=== Setting up systemd user service ==="
+mkdir -p "$HOME/.config/systemd/user"
+cat > "$HOME/.config/systemd/user/act-runner.service" << EOF
+[Unit]
+Description=Gitea Act Runner
+After=network.target
+
+[Service]
+Type=simple
+WorkingDirectory=%h
+ExecStart=%h/bin/act_runner daemon
+Restart=on-failure
+RestartSec=10
+
+[Install]
+WantedBy=default.target
+EOF
+
+systemctl --user daemon-reload
+systemctl --user enable act-runner
+systemctl --user start act-runner
+
+echo ""
+echo "=== Done! ==="
+systemctl --user status act-runner --no-pager | head -8
+echo ""
+echo "Check Gitea → Settings → Actions → Runners to verify."