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/models/domain.py

@@ -2,7 +2,7 @@ from datetime import datetime
 from enum import StrEnum
 from uuid import uuid4
 
-from sqlalchemy import JSON, Boolean, DateTime, Enum, ForeignKey, Integer, String, Text, UniqueConstraint
+from sqlalchemy import JSON, BigInteger, Boolean, DateTime, Enum, ForeignKey, Integer, String, Text, UniqueConstraint
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from app.db.session import Base
@@ -129,6 +129,18 @@ class Node(Base, TimestampMixin):
     cluster: Mapped[Cluster] = relationship()
 
 
+class NodeAgent(Base, TimestampMixin):
+    __tablename__ = "node_agents"
+
+    node_id: Mapped[str] = mapped_column(ForeignKey("nodes.id"), primary_key=True)
+    status: Mapped[str] = mapped_column(String(100), default="not_installed")
+    version: Mapped[str | None] = mapped_column(String(50))
+    last_seen_at: Mapped[datetime | None] = mapped_column(DateTime)
+    last_payload: Mapped[dict | None] = mapped_column(JSON)
+    install_count: Mapped[int] = mapped_column(Integer, default=0)
+    node: Mapped[Node] = relationship()
+
+
 class Workload(Base, TimestampMixin):
     __tablename__ = "workloads"
 
@@ -183,6 +195,23 @@ class IpAddress(Base, TimestampMixin):
     note: Mapped[str | None] = mapped_column(Text)
 
 
+class TrafficFlow(Base, TimestampMixin):
+    __tablename__ = "traffic_flows"
+
+    id: Mapped[str] = mapped_column(String, primary_key=True, default=new_id)
+    node_id: Mapped[str] = mapped_column(ForeignKey("nodes.id"), index=True)
+    source_ip: Mapped[str] = mapped_column(String(100), index=True)
+    destination_ip: Mapped[str] = mapped_column(String(100), index=True)
+    protocol: Mapped[str] = mapped_column(String(20), default="unknown")
+    source_port: Mapped[int | None] = mapped_column(Integer)
+    destination_port: Mapped[int | None] = mapped_column(Integer)
+    bytes: Mapped[int] = mapped_column(BigInteger, default=0)
+    packets: Mapped[int] = mapped_column(BigInteger, default=0)
+    state: Mapped[str | None] = mapped_column(String(100))
+    observed_at: Mapped[datetime | None] = mapped_column(DateTime)
+    raw: Mapped[dict | None] = mapped_column(JSON)
+
+
 class SecurityGroup(Base, TimestampMixin):
     __tablename__ = "security_groups"