backend: make ProcessStats constructor private for strict singleton

- Add private keyword to ProcessStats constructor
- Prevents direct instantiation with `new ProcessStats()`
- Enforces use of `ProcessStats.getInstance()` method
- TypeScript will now catch attempts to bypass singleton pattern

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: haraldschilly

src/packages/backend/process-stats.ts

@@ -47,7 +47,7 @@ export class ProcessStats {
   private pagesize: number;
   private last?: { timestamp: number; processes: Processes };
 
-  constructor(opts?: ProcessStatsOpts) {
+  private constructor(opts?: ProcessStatsOpts) {
     this.procLimit = opts?.procLimit ?? LIMIT;
     this.dbg = opts?.dbg ?? getLogger("process-stats").debug;
     this.init();