feat(sub): expose last subscription fetch time (#6217)

* feat(sub): expose last subscription fetch time

Record successful GET subscription fetches per client and surface the timestamp in the client API and UI.

* fix(frontend): include last subscription fetch in client traffic

* Update sub_fetch_test.go

---------

Co-authored-by: Hermes Agent <hermes-agent@localhost>

frontend/public/openapi.json

@@ -1324,6 +1324,11 @@
             "format": "int64",
             "type": "integer"
           },
+          "lastSubFetch": {
+            "example": 1735680000000,
+            "format": "int64",
+            "type": "integer"
+          },
           "reset": {
             "example": 0,
             "type": "integer"
@@ -1355,6 +1360,7 @@
           "id",
           "inboundId",
           "lastOnline",
+          "lastSubFetch",
           "reset",
           "subId",
           "total",
@@ -3139,6 +3145,7 @@
                           "id": 14825,
                           "inboundId": 1,
                           "lastOnline": 1735680000000,
+                          "lastSubFetch": 1735680000000,
                           "reset": 0,
                           "subId": "i7tvdpeffi0hvvf1",
                           "total": 10737418240,
@@ -7786,6 +7793,7 @@
                     "id": 14825,
                     "inboundId": 1,
                     "lastOnline": 1735680000000,
+                    "lastSubFetch": 1735680000000,
                     "reset": 0,
                     "subId": "i7tvdpeffi0hvvf1",
                     "total": 10737418240,