feat: add interface traffic counters as fallback telemetry when conntrack flows unavailable

Add interface_traffic collection in agent to aggregate VM/LXC network counters by vmid/nic with tap/fwln/fwpr/fwbr interface ranking, implement collect_interface_traffic to select best interface per VM NIC and format as flow-like records with rx/tx bytes/packets, add collect_flow_diagnostics to capture conntrack binary path and kernel bridge/netfilter settings for debugging, update workload_insights endpoint

backend/app/schemas/domain.py

@@ -203,6 +203,7 @@ class AgentHeartbeat(BaseModel):
     uptime_seconds: float | None = None
     loadavg: list[float] = []
     interfaces: list[dict[str, Any]] = []
+    interface_traffic: list[dict[str, Any]] = []
     flows: list[dict[str, Any]] = []
     conntrack: dict[str, Any] = {}
     firewall: dict[str, Any] = {}