feat: add node agent system with heartbeat collection, installer generation, and traffic flow telemetry

Add NodeAgent and TrafficFlow models to track agent status and network flows, implement /agents/heartbeat endpoint to receive interface counters, conntrack flows, firewall status, and nftables ruleset hash from agents, add nexafabric-agent.py Python script to collect host telemetry including VM/LXC interface hints via tap/fwbr regex matching, conntrack flow parsing with protocol/state/byte counters, and pve-firewall status checks,

backend/app/schemas/domain.py

@@ -181,6 +181,34 @@ class NodeRead(OrmModel):
     memory_mb: int
 
 
+class NodeAgentRead(BaseModel):
+    node_id: str
+    status: str
+    version: str | None = None
+    last_seen_at: datetime | None = None
+    install_count: int = 0
+    last_payload: dict[str, Any] | None = None
+
+
+class NodeWithAgentRead(NodeRead):
+    agent: NodeAgentRead | None = None
+
+
+class AgentHeartbeat(BaseModel):
+    version: str
+    node_name: str | None = None
+    collected_at: datetime | None = None
+    hostname: str | None = None
+    kernel: str | None = None
+    uptime_seconds: float | None = None
+    loadavg: list[float] = []
+    interfaces: list[dict[str, Any]] = []
+    flows: list[dict[str, Any]] = []
+    conntrack: dict[str, Any] = {}
+    firewall: dict[str, Any] = {}
+    extra: dict[str, Any] = {}
+
+
 class WorkloadRead(OrmModel):
     id: str
     cluster_id: str