feat(panel): customers stats strip with 3x-ui depletion semantics (round-2 #1)

GET /api/customers gains a summary object (total/online/depleted/
depletingSoon/disabled/enabled) computed server-side; the customers page
renders it as a six-cell stats strip above the toolbar.

Predicates aligned verbatim with 3x-ui client_paging.go:
- depleted = (limit>0 && used>=limit) || (expiry set && expiry<=now),
  counted regardless of status;
- depletingSoon = active && !depleted && (expiry within expireDiff days
  || remaining traffic < trafficDiff GB), thresholds from new settings
  expireDiff/trafficDiff with 3x-ui's factory default 0 (nobody counts
  until configured);
- enabled = status active; disabled = manual suspensions only.
Online reuses the existing 3-minute presence projection.

Golden diff confined to the additive summary keys; summary_test.go
covers every bucket with thresholds off (soon=0) and on (soon=2).

panel-go/golden/golden.json

@@ -1,6 +1,6 @@
 {
   "meta": {
-    "capturedAt": "2026-10-20T20:52:29.987Z",
+    "capturedAt": "2026-10-20T20:56:25.514Z",
     "note": "Go panel golden baseline; see README.md in this directory."
   },
   "sections": {
@@ -105,7 +105,15 @@
               "usedDownlinkBytes": 0,
               "usedUplinkBytes": 0
             }
-          ]
+          ],
+          "summary": {
+            "depleted": 0,
+            "depletingSoon": 0,
+            "disabled": 0,
+            "enabled": 2,
+            "online": 0,
+            "total": 2
+          }
         },
         "status": 200
       },
@@ -1298,7 +1306,15 @@
             "usedDownlinkBytes": 0,
             "usedUplinkBytes": 0
           }
-        ]
+        ],
+        "summary": {
+          "depleted": 0,
+          "depletingSoon": 0,
+          "disabled": 0,
+          "enabled": 3,
+          "online": 1,
+          "total": 3
+        }
       },
       "status": 200
     },