Pins actions/setup-node to v3, skips DB backup when no database exists yet

actions/setup-node@v4's Post step attempts a cache-save that fails hard on
this self-hosted Gitea Actions runner (no GHES cache API support), marking
frontend-prepare as failed and blocking its dependents even though the
actual install/build work succeeds. v3's older cache implementation doesn't
hit this. Also cleans up two stale comments left over from the /admin
MSBuild-target removal.

Also updates the documented backup script to skip cleanly (exit 0) when the
target database doesn't exist yet (MariaDB error 1049) rather than failing
the whole deploy - expected on a brand-new environment's first production
run, before migrations have ever had a chance to create it. Still fails hard
on any other error, so a real backup failure against an existing database
still blocks the deploy as intended.

aidlc-docs/features/gitea-deployment-workflow/operations/deployment/deployment-instructions.md

@@ -556,6 +556,21 @@ fi
 source "$CREDENTIALS_FILE"
 : "${DB_HOST:?}" "${DB_PORT:?}" "${DB_NAME:?}" "${DB_USER:?}" "${DB_PASSWORD:?}"
 
+# A brand-new environment's very first production deploy runs this step before the app has ever
+# started, so before EF Core's migrations have had a chance to create the database — there is
+# nothing to back up yet, and that's fine, not a failure. Distinguish that specific case ("Unknown
+# database", MariaDB error 1049) from every other failure (wrong credentials, network issue,
+# permissions): only the former is safe to skip, since skipping any OTHER error would silently mask
+# a real backup failure against a database that may hold real data.
+USE_ERROR=$(mariadb -h "$DB_HOST" -P "$DB_PORT" -u "$DB_USER" -p"$DB_PASSWORD" -e "USE \`$DB_NAME\`;" 2>&1 >/dev/null) || true
+if echo "$USE_ERROR" | grep -q "Unknown database"; then
+  echo "Database '$DB_NAME' does not exist yet — nothing to back up (expected on a first deploy). Skipping."
+  exit 0
+elif [[ -n "$USE_ERROR" ]]; then
+  echo "Could not verify database '$DB_NAME' exists: $USE_ERROR" >&2
+  exit 1
+fi
+
 BACKUP_DIR="$HOME/backups/slpsoftware/${ENVIRONMENT}"
 mkdir -p "$BACKUP_DIR"
 TIMESTAMP=$(date -u +%Y%m%d%H%M%S)