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/src/generated/schemas.ts

@@ -1298,6 +1298,11 @@ export const SCHEMAS: Record<string, unknown> = {
         "format": "int64",
         "type": "integer"
       },
+      "lastSubFetch": {
+        "example": 1735680000000,
+        "format": "int64",
+        "type": "integer"
+      },
       "reset": {
         "example": 0,
         "type": "integer"
@@ -1329,6 +1334,7 @@ export const SCHEMAS: Record<string, unknown> = {
       "id",
       "inboundId",
       "lastOnline",
+      "lastSubFetch",
       "reset",
       "subId",
       "total",