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

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

frontend/src/schemas/client.ts

@@ -10,6 +10,7 @@ export const ClientTrafficSchema = z.object({
   expiryTime: z.number().optional(),
   enable: z.boolean().optional(),
   lastOnline: z.number().optional(),
+  lastSubFetch: z.number().optional(),
 });
 
 export const ClientRecordSchema = z.object({