Add read-only multi-server API (step 2) + dedupe serverID validation

GET /servers lists registered servers; GET /servers/:id/clients returns
that server's client list (filtered in-handler, store.GetClients isn't
server-scoped yet - that's a later step). Old routes untouched.

Also factors the repeated serverID validation guard in jsondb.go's new
server-scoped methods into one validateServerID() helper, per a code
simplification review.

handler/routes.go

@@ -408,6 +408,54 @@ func GetClient(db store.IStore) echo.HandlerFunc {
 	}
 }
 
+// ListServers handler returns a JSON list of registered servers (step 2 of
+// the multi-server extension - read-only, additive alongside the existing
+// single-server routes).
+func ListServers(db store.IStore) echo.HandlerFunc {
+	return func(c echo.Context) error {
+		servers, err := db.GetServers()
+		if err != nil {
+			return c.JSON(http.StatusInternalServerError, jsonHTTPResponse{
+				false, fmt.Sprintf("Cannot get server list: %v", err),
+			})
+		}
+		return c.JSON(http.StatusOK, servers)
+	}
+}
+
+// GetServerClients handler returns a JSON list of clients belonging to a
+// single server (step 2 of the multi-server extension). Filtering happens
+// in-handler for now since db.GetClients has not yet been switched to a
+// server-scoped signature (that's a later step).
+func GetServerClients(db store.IStore) echo.HandlerFunc {
+	return func(c echo.Context) error {
+		serverID := c.Param("id")
+		if !util.ValidateRecordID(serverID) {
+			return c.JSON(http.StatusBadRequest, jsonHTTPResponse{false, "Please provide a valid server ID"})
+		}
+
+		if _, err := db.GetServerByID(serverID); err != nil {
+			return c.JSON(http.StatusNotFound, jsonHTTPResponse{false, "Server not found"})
+		}
+
+		clientDataList, err := db.GetClients(false)
+		if err != nil {
+			return c.JSON(http.StatusInternalServerError, jsonHTTPResponse{
+				false, fmt.Sprintf("Cannot get client list: %v", err),
+			})
+		}
+
+		var filtered []model.ClientData
+		for _, cd := range clientDataList {
+			if cd.Client.ServerID == serverID {
+				filtered = append(filtered, cd)
+			}
+		}
+
+		return c.JSON(http.StatusOK, filtered)
+	}
+}
+
 // NewClient handler
 func NewClient(db store.IStore) echo.HandlerFunc {
 	return func(c echo.Context) error {