feat: reduce agent payload size with flow truncation and increase nginx body size limit to 16MB

Reduce agent flow_limit from 2000 to 1500, truncate log_excerpt from 500 to 180 characters in firewall log parsing, increase firewall_log_lines_from_files limit from default to max(limit*2, 1000), add compact_agent_payload to truncate flows array to 50 entries with flow_count/flows_truncated metadata, update agent_heartbeat to store compacted payload instead of full dump, add agentFlowCount helper to

nginx/default.conf

@@ -1,8 +1,10 @@
 server {
   listen 80;
   server_name _;
+  client_max_body_size 16m;
 
   location /api/ {
+    client_max_body_size 16m;
     proxy_pass http://api:8000/api/;
     proxy_set_header Host $http_host;
     proxy_set_header X-Real-IP $remote_addr;
@@ -12,6 +14,7 @@ server {
   }
 
   location /agents/ {
+    client_max_body_size 16m;
     proxy_pass http://api:8000/api/v1/agents/;
     proxy_set_header Host $http_host;
     proxy_set_header X-Real-IP $remote_addr;