feat: add subnet edit functionality with internal subnet labeling in workload traffic insights

Add SubnetUpdate schema with optional fields for PATCH operations, implement update_subnet endpoint with validation and audit logging, add subnet_label_for_ip helper to match IPs against known subnets using longest prefix matching, update flow_endpoint_label to show "internal (CIDR)" for traffic within known subnets instead of "external", add DNS servers and DHCP toggle to subnet form UI, implement edit

backend/app/schemas/domain.py

@@ -113,6 +113,14 @@ class SubnetCreate(BaseModel):
     dhcp_enabled: bool = False
 
 
+class SubnetUpdate(BaseModel):
+    network_id: str | None = None
+    cidr: str | None = None
+    gateway: str | None = None
+    dns: list[str] | None = None
+    dhcp_enabled: bool | None = None
+
+
 class SecurityGroupCreate(BaseModel):
     project_id: str | None = None
     name: str