fix: wire update_checks into telemetry-sync + session count fallback

Three bug fixes:
- Telemetry-sync now pings update_checks on successful event sync
  (previously only in gstack-update-check on cache-miss path)
- community-pulse falls back to distinct session_id count when
  update_checks is empty
- Dashboard queries session_id and shows unique session count

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

bin/gstack-telemetry-sync

@@ -118,7 +118,26 @@ HTTP_CODE="$(curl -s -o /dev/null -w '%{http_code}' --max-time 10 \
 # ─── Update cursor on success (2xx) ─────────────────────────
 case "$HTTP_CODE" in
   2*) NEW_CURSOR=$(( CURSOR + COUNT ))
-      echo "$NEW_CURSOR" > "$CURSOR_FILE" 2>/dev/null || true ;;
+      echo "$NEW_CURSOR" > "$CURSOR_FILE" 2>/dev/null || true
+
+      # Ping update_checks (install base proxy)
+      GSTACK_VERSION="$(cat "$GSTACK_DIR/VERSION" 2>/dev/null | tr -d '[:space:]' || echo "unknown")"
+      _OS="$(uname -s | tr '[:upper:]' '[:lower:]')"
+      curl -sf --max-time 5 \
+        -X POST "${ENDPOINT}/update_checks" \
+        -H "Content-Type: application/json" \
+        -H "apikey: ${ANON_KEY}" \
+        -H "Authorization: Bearer ${ANON_KEY}" \
+        -H "Prefer: return=minimal" \
+        -d "{\"gstack_version\":\"$GSTACK_VERSION\",\"os\":\"$_OS\"}" \
+        >/dev/null 2>&1 || true
+
+      # Trigger community backup if community tier
+      BACKUP_CMD="$GSTACK_DIR/bin/gstack-community-backup"
+      if [ "$TIER" = "community" ] && [ -x "$BACKUP_CMD" ]; then
+        "$BACKUP_CMD" 2>/dev/null &
+      fi
+      ;;
 esac
 
 # Update rate limit marker