wiki: Dingtian relay decision, HTTP-over-MQTT, unmanned direction

- dingtian-relay: relay+input controller (4ch on hand). Inputs are decoupled
  from relays (configurable via input_link_relay) — solves the
  access-controller-button-flow blocker the UHPPOTE couldn't. Full protocol from
  the SDK (UDP string control :60001, `00` status parse, input_link_url push,
  multicast discovery). Driver + hardware test still to build.
- dingtian-vs-mqtt: use direct HTTP/UDP now; MQTT skipped (broker = extra infra
  + failure mode + overkill at one-host/few-devices scale) but kept for later
  multi-lane scale.
- autonomous-direction: record the roadmap to fully unmanned (no booth) and how
  it reshapes the threat model (operator-fraud -> unattended-machine threats),
  makes host-in-the-loop entry mandatory, and raises fail-state stakes.
- threat-model: note the unmanned shift. index + log.

gitignore the vendor SDK (dingtian/, 71MB of binaries/examples) — reference
only, protocol captured in the wiki.

wiki/decisions/dingtian-vs-mqtt.md

@@ -0,0 +1,46 @@
+---
+type: decision
+tags: [parking, decision, devices, transport]
+sources: []
+updated: 2026-06-15
+status: settled
+---
+
+# Transport for the Relay Controller: HTTP/UDP now, MQTT parked
+
+**Decision (2026-06-15): use direct HTTP + UDP for the [[dingtian-relay]] controller now. MQTT is
+deliberately skipped, but kept on the radar** for when the system scales.
+
+## Options the device supports
+
+The Dingtian relay board speaks several protocols: Dingtian string (UDP/TCP), Dingtian binary
+(UDP, optional multicast/password), **HTTP CGI**, **HTTP input-link push** (`input_link_url`),
+**Modbus** (RTU/TCP/ASCII), and **MQTT**.
+
+## Why not MQTT (yet)
+
+- **A broker is new infrastructure** on a deliberately **single-purpose hardened appliance**
+  ([[disk-os-hardening]]) — another service to install, secure, supervise, and keep alive.
+- **Extra failure mode on the critical path.** Today host→UDP→relay. MQTT inserts a broker on both
+  control and event paths; if it stalls, the lane stalls — and there are 3 processes to debug, not 2.
+- **Doesn't fit [[offline-first]] for this scale.** MQTT earns its keep with *many* devices/consumers
+  and intermittent links. Here it's **one host + a few devices on one isolated LAN, metres apart** —
+  request/response control + a single input event, no fleet.
+- **The device's MQTT input publish is periodic** ("default every 30 s"), so it's not even a clean
+  on-press event without relying on unverified on-change behaviour.
+
+## Why HTTP/UDP fits
+
+- **Relay control:** direct **UDP string protocol** (port 60001) — `11`=relay1 on, `21`=off,
+  `T1`=toggle, `11*`=jog/pulse. No deps, no broker.
+- **Input/button events:** the device's **`input_link_url`** can **HTTP POST to the host backend
+  when an input fires** — real push, device calls our existing Fastify server directly, no broker.
+  (Polling `00` status over UDP every ~50 ms is the self-contained fallback.)
+- Fewest moving parts; matches the local same-origin model already in use.
+
+## When to revisit MQTT
+
+If the system grows to **many lanes / many controllers**, or multiple subsystems (LPR, payment,
+signage) all need to share events, a broker becomes a worthwhile central event bus. That aligns
+with the [[autonomous-direction|unmanned]] roadmap at multi-lane scale — re-evaluate then. Until
+then, direct HTTP/UDP wins on simplicity and reliability.