Restore LXC memory fallback to the cluster-resources listing

Real PVE guest RRD responses carry only the cache-inclusive mem/maxmem
columns; the cache-aware memused/memavailable columns exist only in node
RRD, so the LXC RRD branches can never match a live response. Removing
the cluster-resources fallback in bf67ba920 therefore left every running
LXC reporting unavailable memory, rendered as 0% (#1634).

Running containers now fall back to the listing value under the
low-trust cluster-resources source when cache-aware RRD evidence is
absent; unavailable remains reserved for running containers with no
listing evidence at all.

Fixes #1634

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/monitoring/memory_trust_characterization_test.go

@@ -1053,7 +1053,7 @@ func TestHandleClusterContainerResourceMemoryTrustCharacterization(t *testing.T)
 			wantStatus:   "running",
 		},
 		{
-			name: "running LXC without RRD memory stays unknown",
+			name: "running LXC without RRD memory falls back to cluster resources (#1634)",
 			res: proxmox.ClusterResource{
 				ID:     "lxc/204",
 				Type:   "lxc",
@@ -1065,6 +1065,22 @@ func TestHandleClusterContainerResourceMemoryTrustCharacterization(t *testing.T)
 				Mem:    7 * gib,
 				MaxCPU: 4,
 			},
+			wantSource: "cluster-resources",
+			wantUsed:   7 * gib,
+			wantStatus: "running",
+		},
+		{
+			name: "running LXC without RRD memory or listing value stays unknown",
+			res: proxmox.ClusterResource{
+				ID:     "lxc/206",
+				Type:   "lxc",
+				Node:   "node1",
+				Name:   "ct-206",
+				Status: "running",
+				VMID:   206,
+				MaxMem: 8 * gib,
+				MaxCPU: 4,
+			},
 			wantSource:  "unavailable",
 			wantStatus:  "running",
 			wantUnknown: true,