Mitra availability: read paths respect require_mitra_ping=false

When the operator sets require_mitra_ping=false, the auto-offline sweep
early-returns (by design — "don't gate online status on heartbeat
freshness"). The three Valkey read paths still gated on heartbeat
freshness anyway, which trapped the system: sweep won't remove the
mitra from mitras:online, but readers reject them as stale. The customer
CTA stayed permanently disabled with no recovery.

Fix all three to skip the heartbeat-freshness check when require_ping
is off, matching the sweep's contract:
- computeAvailabilityFromValkey (customer beacon)
- isMitraReachable (extension service)
- findAvailableMitrasFromValkey (pairing candidate finder)

The Postgres fallbacks already did the right thing (is_online only,
no heartbeat compare); this aligns the Valkey hot path.

Also: PATCH /internal/config/mitra-ping now publishes config:invalidate
for require_mitra_ping and mitra_stale_after_seconds, and the subscriber
in mitra-status.service was widened to listen for both. Flipping the
toggle in CC now busts the 10s availability snapshot immediately instead
of waiting out the TTL.

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

backend/src/services/pairing.service.js

@@ -83,7 +83,7 @@ const notifyCustomer = async (customerId, data) => {
 // Postgres fallback runs if any Valkey op throws (full JOIN as before).
 const findAvailableMitrasFromValkey = async () => {
   const { max_customers_per_mitra } = await getMaxCustomersPerMitra()
-  const { stale_after_seconds } = await getMitraPingConfig()
+  const { require_ping, stale_after_seconds } = await getMitraPingConfig()
 
   const candidates = await valkey.sdiff(VK_MITRAS_ONLINE, VK_MITRAS_DEACTIVATED)
   if (!candidates.length) return []
@@ -91,17 +91,23 @@ const findAvailableMitrasFromValkey = async () => {
   const pipe = valkey.pipeline()
   for (const id of candidates) {
     pipe.get(vkCapacityKey(id))
-    pipe.get(vkHeartbeatKey(id))
+    if (require_ping) pipe.get(vkHeartbeatKey(id))
   }
   const results = await pipe.exec()
+  const stride = require_ping ? 2 : 1
 
   const cutoff = Date.now() - stale_after_seconds * 1000
   const eligible = []
   for (let i = 0; i < candidates.length; i++) {
-    const capacity = Number(results[i * 2][1] ?? 0)
-    const heartbeat = results[i * 2 + 1][1]
+    const capacity = Number(results[i * stride][1] ?? 0)
     if (capacity >= max_customers_per_mitra) continue
-    if (!heartbeat || Date.parse(heartbeat) < cutoff) continue
+    // See computeAvailabilityFromValkey in mitra-status.service.js: when the
+    // ping requirement is off, the sweep is off too, so we don't gate
+    // candidate selection on heartbeat freshness here either.
+    if (require_ping) {
+      const heartbeat = results[i * stride + 1][1]
+      if (!heartbeat || Date.parse(heartbeat) < cutoff) continue
+    }
     eligible.push({ id: candidates[i], active_session_count: capacity })
   }
   return eligible