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/zod.ts

@@ -340,6 +340,7 @@ export const ClientTrafficSchema = z.object({
   id: z.number().int(),
   inboundId: z.number().int(),
   lastOnline: z.number().int(),
+  lastSubFetch: z.number().int(),
   reset: z.number().int(),
   subId: z.string(),
   total: z.number().int(),