[Enhancement] Add safe operator upgrade controls

[Al-assad]

Summary

• support pausing reconciliation per DorisCluster with apache.org.doris/reconcile-paused=true
• preserve deletion cleanup for paused clusters and accept the legacy SelectDB annotation
• add a configurable --leader-election-id while preserving the current default
• document a safe upgrade workflow and update direct deployment manifests

Testing

• go test ./cmd/operator/conf
• go test ./pkg/controller -run Test(ReconcilePausedDorisCluster|PausedDorisClusterDeletionStillCleansResources)$ -count=1
• go test ./cmd/operator/... -run ^$
• git diff --check
• parsed the three updated operator YAML manifests

No Helm files are changed.