fix(ci): guard against empty HOST_IP in smoke test

If `ip route show default` returns no output the old code passed
an empty string to curl --resolve, producing a confusing error 6
("couldn't resolve host") with no indication that gateway detection
had failed.  The new guard exits immediately with a clear message.

Addresses review concern raised by Tobias Wendt.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/nightly.yml

@@ -170,6 +170,7 @@ jobs:
           HOST="staging.raddatz.cloud"
           URL="https://$HOST"
           HOST_IP=$(ip route show default | awk '/default/ {print $3}')
+          [ -n "$HOST_IP" ] || { echo "ERROR: could not detect Docker bridge gateway via 'ip route'"; exit 1; }
           RESOLVE="--resolve $HOST:443:$HOST_IP"
           echo "Smoke test: $URL (pinned to $HOST_IP via bridge gateway)"
           curl -fsS $RESOLVE --max-time 10 "$URL/login" -o /dev/null

.gitea/workflows/release.yml

@@ -114,6 +114,7 @@ jobs:
           HOST="archiv.raddatz.cloud"
           URL="https://$HOST"
           HOST_IP=$(ip route show default | awk '/default/ {print $3}')
+          [ -n "$HOST_IP" ] || { echo "ERROR: could not detect Docker bridge gateway via 'ip route'"; exit 1; }
           RESOLVE="--resolve $HOST:443:$HOST_IP"
           echo "Smoke test: $URL (pinned to $HOST_IP via bridge gateway)"
           curl -fsS $RESOLVE --max-time 10 "$URL/login" -o /dev/null