docs(wiki): camera 503/stream, alarm URL helper, reader ICMP liveness

- lpr-camera.md: "503 Device Busy" can be PERSISTENT (main-stream saturation on
  the G3H) — the real fix is sub-stream selection, not just retry.
- device-status-monitoring.md: QR reader health was false-healthy (hardcoded
  "ready") until the ICMP-ping fix; document the push-device monitoring model.
- log entries for both 2026-06-26 sessions.

Claude-Session: https://claude.ai/code/session_01Xcm6ikLgGoCxxHrxtjkk5V

wiki/concepts/device-status-monitoring.md

@@ -2,7 +2,7 @@
 type: concept
 tags: [parking, device, monitoring, reliability, ui]
 sources: []
-updated: 2026-06-18
+updated: 2026-06-26
 status: open
 ---
 
@@ -25,6 +25,15 @@ talks only to the adapter interfaces ([[device-adapter-pattern]]), never a drive
 - **Relays / readers / cameras** → the generic `Device.healthCheck()` **reachability** probe every
   adapter implements (`ready | degraded | offline`). This is presence/up-ness, not a deep fault
   model — a relay either answers or it doesn't.
+  > **Reader health was a LIE until 2026-06-26.** The QR-reader adapter (a PUSH device: it GETs our
+  > backend on each scan and exposes **no TCP port**) had a hardcoded `healthCheck → { ready, "stub" }`,
+  > so two genuinely-offline readers still showed **green**. A push device that's silent is
+  > indistinguishable from a dead one — so claiming `ready` unconditionally is the worst failure
+  > (false-healthy). Fix: an **optional reader IP** (monitor-only; scans still resolve by serial) +
+  > an **unprivileged ICMP ping** (`drivers/icmp.ts`, shells `/bin/ping` in SOCK_DGRAM mode — no
+  > CAP_NET_RAW, no native dep; the booth compose sets `net.ipv4.ping_group_range`). Reply → `ready`,
+  > no reply → `offline`; **no IP set → `degraded` ("set IP to monitor")**, never a false green.
+  > Verified on hardware: pinged the real readers on the device VLAN. See [[gee-qr-er80]].
 
 Both collapse to one **traffic-light**: `ready | degraded | offline`, plus a `detail` string. Fail
 **toward "there's a problem"**, never false-healthy: a probe that throws or times out reads