fix(error-handling): surface silent errors across the codebase (#326)

Add console.warn/console.error logging to 22 silent catch blocks across
10 files. Errors in cleanup, migrations, SSO, fleet snapshots, shutdown,
and validation are now visible in logs. ENOENT guards added to
file-system catches to distinguish missing files from permission errors.
No control flow changes.

backend/src/services/MonitorService.ts

@@ -180,7 +180,9 @@ export class MonitorService {
                                 totalReclaimableBytes += bytes;
                             }
                         }
-                    } catch (e) { }
+                    } catch (e) {
+                        console.warn('[MonitorService] Failed to parse Docker system df output:', e);
+                    }
                 }
 
                 const reclaimGb = totalReclaimableBytes / (1024 * 1024 * 1024);