Mitra ping: decouple stale-after from app cadence

Splits the single mitra_ping_interval_seconds config (which conflated
"how often the app pings" with "how long until offline" through a
hidden ×3 multiplier) into two orthogonal knobs:

- mitra_stale_after_seconds (CC-tunable, app_config DB row): the
  operator-facing offline threshold. What you set is what you get —
  no multiplier. Default 45s (preserves today's effective grace at
  the legacy 15s ping default).
- MITRA_HEARTBEAT_CADENCE_SECONDS (env var, default 30s): how often
  the mitra app sends a heartbeat. Backend-fixed per deployment;
  surfaced to the mitra app via /api/mitra/status.

Backend:
- config.service: getMitraPingConfig returns the new tuple
  {require_ping, stale_after_seconds, heartbeat_cadence_seconds}.
  Env parser handles blank/non-numeric → 30 fallback.
- mitra-status.service::autoOfflineStaleMitras drops the *3 and uses
  stale_after_seconds directly.
- mitra-status.service::getStatus returns heartbeat_cadence_seconds
  instead of ping_interval_seconds.
- /internal/config/mitra-ping PATCH validates
  stale_after_seconds >= cadence, returns 422 with a clear message
  ("stale_after_seconds must be a number >= heartbeat cadence (30s)").
- migrate.js: adds mitra_stale_after_seconds default 45. The old
  mitra_ping_interval_seconds key is left in place (vestigial) —
  no live code reads it; safe to drop after one release.

Mitra app:
- status_notifier reads heartbeat_cadence_seconds, uses it directly
  as the Timer.periodic interval. Defaults to 30s if missing (older
  backend safety).

Control center:
- SettingsPage: renames "Interval Ping" → "Ambang offline", input
  min={heartbeat_cadence_seconds}, shows the cadence as a read-only
  value with explanation that it's env-controlled.

Verified end-to-end on dev backend:
- GET /api/mitra/status returns {…, heartbeat_cadence_seconds: 30}
- GET /internal/config/mitra-ping returns {require_ping,
  stale_after_seconds: 45, heartbeat_cadence_seconds: 30}
- PATCH with stale_after_seconds=20 → 422 with cadence message
- PATCH with stale_after_seconds=120 → 200, persisted
- Env override (=60, blank, "foo") parses correctly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/routes/internal/config.routes.js

@@ -9,7 +9,7 @@ import {
   getFreeTrialConfig, setFreeTrialConfig,
   getExtensionTimeoutConfig, setExtensionTimeoutConfig,
   getEarlyEndConfig, setEarlyEndConfig,
-  getMitraPingConfig, setMitraPingConfig,
+  getMitraPingConfig, setMitraPingConfig, getMitraHeartbeatCadenceSeconds,
   getSensitivityConfig, setSensitivityConfig,
   getPaymentSessionTimeoutMinutes, setPaymentSessionTimeoutMinutes,
   getReturningChatConfirmationTimeoutSeconds, setReturningChatConfirmationTimeoutSeconds,
@@ -173,14 +173,23 @@ export const internalConfigRoutes = async (app) => {
   app.patch('/mitra-ping', {
     preHandler: [authenticate, attachCcUser, requirePermission('config', 'update')],
   }, async (request, reply) => {
-    const { require_ping, ping_interval_seconds } = request.body ?? {}
+    const { require_ping, stale_after_seconds } = request.body ?? {}
     if (require_ping !== undefined && typeof require_ping !== 'boolean') {
       return reply.code(422).send({ success: false, error: { code: 'VALIDATION_ERROR', message: 'require_ping must be a boolean' } })
     }
-    if (ping_interval_seconds !== undefined && (typeof ping_interval_seconds !== 'number' || ping_interval_seconds < 5)) {
-      return reply.code(422).send({ success: false, error: { code: 'VALIDATION_ERROR', message: 'ping_interval_seconds must be a number >= 5' } })
+    if (stale_after_seconds !== undefined) {
+      const cadence = getMitraHeartbeatCadenceSeconds()
+      if (typeof stale_after_seconds !== 'number' || stale_after_seconds < cadence) {
+        return reply.code(422).send({
+          success: false,
+          error: {
+            code: 'VALIDATION_ERROR',
+            message: `stale_after_seconds must be a number >= heartbeat cadence (${cadence}s)`,
+          },
+        })
+      }
     }
-    const config = await setMitraPingConfig({ require_ping, ping_interval_seconds })
+    const config = await setMitraPingConfig({ require_ping, stale_after_seconds })
     return reply.send({ success: true, data: config })
   })