Add load-aware primary node placement webhook for volume creation

[boddumanohar]

Summary

• New PVC-mutating admission webhook (SimplyblockVolumePlacementInjector) that computes the least-loaded eligible storage node for a new volume's primary, reusing the same latency-deviation signal the auto-rebalancer (Automatic Rebalancing of Volumes (same Storage Cluster) #130) already computes, and stamps it onto the PVC as the simplyblock.io/host-id annotation — read by spdk-csi's CreateVolume path (zero spdk-csi changes needed).
• Extends autobalancing.StorageNodeSelector with SelectBestNode, ranking eligible nodes by current latency deviation (no source-node/threshold gate, unlike migration-target selection).
• Extends webapi.StorageNodeInfo with Lvols/LvolsMax (already returned by the backend, just not previously mapped) and IsSecondary (new backend field — see companion PR below) to filter out at-capacity and secondary nodes.
• Fails open on every skip/error condition (feature not enabled for a cluster, backend/Prometheus unreachable, no eligible node) — falls back to sbcli's existing weighted-random placement exactly as it works today.
• Full design: operator/docs/designs/design-primary-node-placement.md.

Dependency

Requires simplyblock/sbcli#1175 (exposes is_secondary on the v2 storage-nodes API) to be merged/deployed for the secondary-node filter to work; until then that filter defaults to false (fail-safe, not fail-open on that specific check) since the JSON field will simply be absent.

Test plan

• go build ./... and go vet ./...
• make test (includes envtest-based controller suite) — all packages pass
• make lint (golangci-lint) — 0 issues
• New unit tests for the webhook: skip conditions (annotation already set, no/invalid StorageClass, cluster not found, rebalancing disabled/misconfigured), full node-selection path against fake webapi + Prometheus servers (verifies eligibility filtering + coolest-node ranking), no-eligible-node fallback
• make manifests / make operator-build-installer regenerated (webhook registration only; RBAC already covered by existing markers)

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