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/services/config.service.js

@@ -128,18 +128,38 @@ export const getEarlyEndConfig = async () => {
   }
 }
 
-// --- Phase 3.1: Mitra Ping Config ---
+// --- Mitra reachability config ---
+//
+// Two separate concerns, deliberately decoupled:
+//   - heartbeat_cadence_seconds: how often the mitra app sends a heartbeat.
+//     Fixed per backend deployment via the MITRA_HEARTBEAT_CADENCE_SECONDS
+//     env (default 30). The mitra app reads this from /api/mitra/status and
+//     uses it directly as its Timer.periodic interval.
+//   - stale_after_seconds: how long the backend tolerates silence before
+//     marking a mitra offline. DB-stored, CC-tunable. Must be >= the
+//     heartbeat cadence (CC PATCH validates this).
+//
+// `require_ping` stays as the master switch — when false, the auto-offline
+// sweep is skipped entirely and mitras stay online forever once they toggle.
+
+export const getMitraHeartbeatCadenceSeconds = () => {
+  const raw = process.env.MITRA_HEARTBEAT_CADENCE_SECONDS
+  if (!raw || raw.trim() === '') return 30
+  const parsed = Number.parseInt(raw, 10)
+  return Number.isFinite(parsed) && parsed >= 5 ? parsed : 30
+}
 
 export const getMitraPingConfig = async () => {
   const [requireRow] = await sql`SELECT value FROM app_config WHERE key = 'require_mitra_ping'`
-  const [intervalRow] = await sql`SELECT value FROM app_config WHERE key = 'mitra_ping_interval_seconds'`
+  const [staleRow] = await sql`SELECT value FROM app_config WHERE key = 'mitra_stale_after_seconds'`
   return {
     require_ping: requireRow?.value?.value ?? true,
-    ping_interval_seconds: intervalRow?.value?.value ?? 15,
+    stale_after_seconds: staleRow?.value?.value ?? 45,
+    heartbeat_cadence_seconds: getMitraHeartbeatCadenceSeconds(),
   }
 }
 
-export const setMitraPingConfig = async ({ require_ping, ping_interval_seconds }) => {
+export const setMitraPingConfig = async ({ require_ping, stale_after_seconds }) => {
   if (require_ping !== undefined) {
     await sql`
       INSERT INTO app_config (key, value, updated_at)
@@ -147,10 +167,10 @@ export const setMitraPingConfig = async ({ require_ping, ping_interval_seconds }
       ON CONFLICT (key) DO UPDATE SET value = EXCLUDED.value, updated_at = NOW()
     `
   }
-  if (ping_interval_seconds !== undefined) {
+  if (stale_after_seconds !== undefined) {
     await sql`
       INSERT INTO app_config (key, value, updated_at)
-      VALUES ('mitra_ping_interval_seconds', ${sql.json({ value: ping_interval_seconds })}, NOW())
+      VALUES ('mitra_stale_after_seconds', ${sql.json({ value: stale_after_seconds })}, NOW())
       ON CONFLICT (key) DO UPDATE SET value = EXCLUDED.value, updated_at = NOW()
     `
   }