feat: add background stats server daemon lifecycle

Implement `subminer stats -b` to start a background stats daemon and
`subminer stats -s` to stop it, with PID-based process lifecycle
management, single-instance lock bypass for daemon mode, and automatic
reuse of running daemon instances.

launcher/types.ts

@@ -112,6 +112,8 @@ export interface Args {
   jellyfinDiscovery: boolean;
   dictionary: boolean;
   stats: boolean;
+  statsBackground?: boolean;
+  statsStop?: boolean;
   statsCleanup?: boolean;
   statsCleanupVocab?: boolean;
   statsCleanupLifetime?: boolean;