perf(worker): 按实例内存上限扩展异步容量

将策略推导出的集群目标与单 Worker 内存安全上限分离，避免失活实例的全部容量转移到单个 2 GiB Pod。

生产环境单实例上限和媒体物化并发设为 24；两实例正常时总容量由 16 提升至 48，后续可通过增加 Worker 节点继续扩展。新增 0093 迁移保存实例容量上限，并兼容滚动发布中的旧实例。

风险：更高媒体并发会增加 Worker 和节点内存压力，保留单实例 24 的硬边界并由持续监控观察 RSS、OOM、队列与节点余量。

验证：go test ./... -count=1；go vet ./...；真实 PostgreSQL 分配与故障转移测试；迁移安全检查；kubectl kustomize；gofmt -l；git diff --check。

apps/api/internal/store/worker_registry_test.go

@@ -0,0 +1,44 @@
+package store
+
+import "testing"
+
+func TestAllocateWorkerCapacitiesHonorsInstanceLimits(t *testing.T) {
+	workers := []activeWorkerCapacity{
+		{InstanceID: "worker-a", CapacityLimit: 24},
+		{InstanceID: "worker-b", CapacityLimit: 24},
+	}
+	allocations, global := allocateWorkerCapacities(workers, 310)
+	if global != 48 || allocations["worker-a"] != 24 || allocations["worker-b"] != 24 {
+		t.Fatalf("allocations=%v global=%d, want 24/24 and 48", allocations, global)
+	}
+}
+
+func TestAllocateWorkerCapacitiesBalancesBelowLimits(t *testing.T) {
+	workers := []activeWorkerCapacity{
+		{InstanceID: "worker-a", CapacityLimit: 24},
+		{InstanceID: "worker-b", CapacityLimit: 24},
+	}
+	allocations, global := allocateWorkerCapacities(workers, 5)
+	if global != 5 || allocations["worker-a"] != 3 || allocations["worker-b"] != 2 {
+		t.Fatalf("allocations=%v global=%d, want 3/2 and 5", allocations, global)
+	}
+}
+
+func TestAllocateWorkerCapacitiesKeepsSurvivorBounded(t *testing.T) {
+	workers := []activeWorkerCapacity{{InstanceID: "worker-a", CapacityLimit: 24}}
+	allocations, global := allocateWorkerCapacities(workers, 310)
+	if global != 24 || allocations["worker-a"] != 24 {
+		t.Fatalf("allocations=%v global=%d, want survivor capped at 24", allocations, global)
+	}
+}
+
+func TestAllocateWorkerCapacitiesSupportsUnequalLimits(t *testing.T) {
+	workers := []activeWorkerCapacity{
+		{InstanceID: "worker-a", CapacityLimit: 1},
+		{InstanceID: "worker-b", CapacityLimit: 4},
+	}
+	allocations, global := allocateWorkerCapacities(workers, 5)
+	if global != 5 || allocations["worker-a"] != 1 || allocations["worker-b"] != 4 {
+		t.Fatalf("allocations=%v global=%d, want 1/4 and 5", allocations, global)
+	}
+}