Add server-registry store methods with record-ID validation (step 1)

New IStore methods for a per-server registry (GetServers, GetServerByID,
CreateServer, DeleteServer, GetServerSettings/SaveServerSettings,
GetServerHashes/SaveServerHashes), all additive - existing single-server
methods untouched. Adds util.ValidateRecordID/ValidateInterfaceName and
applies them to every new method taking a server ID, closing a path-
traversal gap before serverID is ever driven by user input (flagged by
a security review pass).

store/store.go

@@ -27,4 +27,12 @@ type IStore interface {
 	GetPath() string
 	SaveHashes(hashes model.ClientServerHashes) error
 	GetHashes() (model.ClientServerHashes, error)
+	GetServers() ([]model.Server, error)
+	GetServerByID(serverID string) (model.Server, error)
+	CreateServer(server model.Server) error
+	DeleteServer(serverID string) error
+	GetServerSettings(serverID string) (model.ServerSetting, error)
+	SaveServerSettings(serverID string, settings model.ServerSetting) error
+	GetServerHashes(serverID string) (model.ClientServerHashes, error)
+	SaveServerHashes(serverID string, hashes model.ClientServerHashes) error
 }