feat(sub): add opt-in month-end expiry presentation (#6517)

Offer monthly calendar subscriptions an explicit last-valid-second display
without moving their real billing boundary or spending renewal allowances.

Keep the option off by default and limit conversion to a shared fixed
day-1 midnight cutoff at an actual month transition in the panel timezone.
Use the authoritative client calendar mode when aggregating node traffic,
and share the header formatter across raw, JSON, and Clash exports.

Expose the setting in the existing settings API/UI, regenerate its schemas,
and document that clients may report expiry one second early or format the
date differently in another timezone. Add HTTP, settings, and DST coverage.
Stored deadlines, access enforcement, info/remark expiry values, and renewal
accounting remain unchanged.

Refs: #6516

Co-authored-by: JacktheRanger <219502738+JacktheRanger@users.noreply.github.com>

internal/web/translation/es-ES.json

@@ -1493,6 +1493,8 @@
           "note": "Los balanceadores leastPing/leastLoad siempre llevan un burstObservatory. Este interruptor personaliza sus parámetros de probe — apágalo para usar los valores predeterminados integrados. Los cambios se aplican tras reiniciar el panel."
         }
       },
+      "subCalendarExpireInclusive": "Month-end subscription expiry display",
+      "subCalendarExpireInclusiveDesc": "Default off. For clients renewing on day 1 with a midnight cutoff in the panel timezone, report the previous month's last second in subscription expiry headers. Actual cutoff and renewals are unchanged. Clients use their own timezone and may consider the subscription expired one second early.",
       "subInfoNodeEnable": "Configuración de información separada / Nodo ficticio",
       "subInfoNodeEnableDesc": "Si está activado, la información de tráfico y días restantes se muestra en un nodo SOCKS ficticio arriba, y las suscripciones caducadas o agotadas devuelven solo el estado.",
       "subExpiredTemplate": "Plantilla de expirado",