feat: show Ubuntu phased updates apart and leave them out of pending counts

apt lists phased updates as upgradable while an upgrade defers them until
Ubuntu selects the host, so a freshly patched server kept reporting pending
updates. The agent now flags them; the server stores the flag and leaves them
out of patch run counts, and the server page shows them in their own section.

server/internal/models/server.go

@@ -10,6 +10,9 @@ type PackageUpdate struct {
 	Name           string `bson:"name" json:"name"`
 	CurrentVersion string `bson:"current_version,omitempty" json:"current_version,omitempty"`
 	NewVersion     string `bson:"new_version" json:"new_version"`
+	// Phased is an Ubuntu phased update the host is not yet selected for: apt
+	// lists it but an upgrade defers it, so pending counts leave it out.
+	Phased bool `bson:"phased,omitempty" json:"phased,omitempty"`
 }
 
 type CPUInfo struct {