Phase 2b: server/client timeouts, atomic state writes, log mutex, panic and shutdown fixes

- http.Server with ReadHeaderTimeout/IdleTimeout (no read/write timeouts: /stream/ is long-lived)
- shared outbound clients: providerHTTPClient (5m), apiHTTPClient (30s), imgcache client (30s)
- imgcache: download outside the lock, per-item helper, cache URL uses the file name not the fs path
- writeFileAtomic (temp + fsync + rename) for settings/xepg/pms/urls/authentication JSON
- one package-level logMu for WebScreenLog and notifications; ring buffer keeps the newest lines
- notifications evict the oldest instead of random map entries
- xepg XMLTV file removal rebuilt after the loop; data.go range-mutation removed
- API handler returns after error body; WS request/response fresh per command
- checked type assertions in data/backup/provider/screen
- SIGINT/SIGTERM handled in main via src.Shutdown(); fatal paths exit 1

src/security.go

@@ -124,12 +124,7 @@ func maskSettings(s SettingsStruct) SettingsStruct {
 // writePrivateFile : like writeByteToFile but readable only by the owner.
 func writePrivateFile(file string, data []byte) error {
 
-	var filename = getPlatformFile(file)
-
-	if err := os.WriteFile(filename, data, 0600); err != nil {
-		return err
-	}
-
-	// WriteFile keeps the mode of an existing file; tighten it.
-	return os.Chmod(filename, 0600)
+	// The temp file is chmod'ed before the rename, so an existing
+	// world-readable file is replaced by a 0600 one.
+	return writeFileAtomic(getPlatformFile(file), data, 0600)
 }