feat: add eBPF flow collector with helper binary contract and agent integration

Add EBPF_HELPER_CONTRACT.md documenting helper binary invocation with --json/--limit/--duration/--interfaces parameters and expected JSON output format with flows/diagnostics, implement collect_ebpf_flows to invoke helper binary with configurable timeout/window/interfaces and normalize flow fields (source_ip/destination_ip/protocol/ports/packets/bytes/state), add executable_exists and flow_int helpers for binary validation

backend/app/schemas/domain.py

@@ -238,6 +238,7 @@ class AgentHeartbeat(BaseModel):
     interfaces: list[dict[str, Any]] = []
     interface_traffic: list[dict[str, Any]] = []
     flows: list[dict[str, Any]] = []
+    ebpf_flows: list[dict[str, Any]] = []
     conntrack: dict[str, Any] = {}
     firewall: dict[str, Any] = {}
     extra: dict[str, Any] = {}