[Task]: Remove deployment sizing. keep per-project quota

Description

The operator previously mixed two independent "plan" concepts:

• RegistryBackend.spec.plan: Sized the Harbor deployment itself.
• Registry.spec.plan: Sizes an individual project's storage quota.

Because the operator no longer manages deployment infrastructure, RegistryBackend.spec.plan is obsolete. Removing it resolves a major source of design confusion and eliminates dead code across the operator and documentation.

Acceptance criteria

1. Delete Autoscale Code: Delete internal/controller/autoscale.go and autoscale_test.go, including helper functions computeEffectivePlan, largerPlan, and planOrder.
2. Remove PVC Management: Remove RegistryBackendReconciler.ensureStorageSize and harborReleaseSelector (the operator does not manage PVCs).
3. Purge RegistryBackend Sizing Fields: Remove all backend sizing fields in the CRD (aligned with issue 1) and verify no dead references remain:

• spec.plan
• spec.autoscale
• status.effectivePlan
• status.committedStorageBytes
• status.usedStorageBytes
• status.unlimitedProjectCount

4. Preserve Registry.spec.plan: Ensure Registry.spec.plan continues to enforce storage quotas via EnsureProjectQuota and automatically updates on reconcile when modified.
5. Maintain Quota Mapping: Keep projectQuotaGi allocations unchanged:
6. Update Docs: Update user-facing documentation to reference only the single Registry.spec.plan concept.

Verification

Existing quota-convergence tests pass without errors
Running make manifests produces no diff.

Start with internal/controller/autoscale.go and autoscale_test.go, then trace RegistryBackendReconciler.ensureStorageSize, harborReleaseSelector, and EnsureProjectQuota. Check the CRD and user-facing documentation for every listed backend sizing field, preserving Registry.spec.plan quota behavior. Run the quota-convergence tests and make manifests; done means no manifest diff and no dead sizing references.