feat(rack-planner): implement port-to-port connections (patch cables) with dynamic SVG visualization layer

prisma/schema.prisma

@@ -53,6 +53,23 @@ model Port {
   nativeVlan Int?
   vlans      PortVlan[]
   notes      String?
+
+  // Connections — port can be source or target of a patch cable
+  sourceConnections Connection[] @relation("SourcePort")
+  targetConnections Connection[] @relation("TargetPort")
+}
+
+model Connection {
+  id           String   @id @default(cuid())
+  fromPortId   String
+  fromPort     Port     @relation("SourcePort", fields: [fromPortId], references: [id], onDelete: Cascade)
+  toPortId     String
+  toPort       Port     @relation("TargetPort", fields: [toPortId], references: [id], onDelete: Cascade)
+  color        String?  // Optional custom cable color
+  label        String?  // Optional cable label (e.g. "Cable #104")
+  createdAt    DateTime @default(now())
+
+  @@unique([fromPortId, toPortId])
 }
 
 model Vlan {