hub: add additional validation checks for custom api routes

- Validate the user is assigned to system in authenticated routes where
the user passes in system ID. This protects against a somewhat
impractical scenario where an authenticated user cracks a random 15
character alphanumeric ID of a system that doesn't belong to them via
web API.
- Validate that systemd service exists in database before requesting
service details from agent. This protects against authenticated users
getting unit properties of services that aren't explicitly monitored.
- Refactor responses in authenticated routes to prevent enumeration of
other users' random 15 char system IDs.

internal/hub/hub.go

@@ -9,7 +9,6 @@ import (
 	"net/url"
 	"os"
 	"path"
-	"regexp"
 	"strings"
 
 	"github.com/henrygd/beszel/internal/alerts"
@@ -38,8 +37,6 @@ type Hub struct {
 	appURL string
 }
 
-var containerIDPattern = regexp.MustCompile(`^[a-fA-F0-9]{12,64}$`)
-
 // NewHub creates a new Hub instance with default configuration
 func NewHub(app core.App) *Hub {
 	hub := &Hub{App: app}