feat(watcher): log auth field changes on reload

Cache parsed auth contents and compute redacted diffs for prefix, proxy_url,
and disabled when auth files are added or updated.

internal/watcher/watcher.go

@@ -38,6 +38,7 @@ type Watcher struct {
 	reloadCallback    func(*config.Config)
 	watcher           *fsnotify.Watcher
 	lastAuthHashes    map[string]string
+	lastAuthContents  map[string]*coreauth.Auth
 	lastRemoveTimes   map[string]time.Time
 	lastConfigHash    string
 	authQueue         chan<- AuthUpdate