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/autonomous-direction.md

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+---
+type: decision
+tags: [parking, direction, roadmap]
+sources: []
+updated: 2026-06-15
+status: open
+---
+
+# Project Direction: Toward Fully Autonomous (Unmanned)
+
+> Stated goal (2026-06-15): the system will evolve to **fully automatic operation — no human
+> operator, no booth at all**. Recorded because "unmanned" is an architectural force that shapes
+> several existing decisions, not just a feature.
+
+## What "unmanned" changes
+
+- **Threat model shift.** The original primary adversary was *"the legitimate operator at the
+  booth"* ([[threat-model]]). Remove the operator and that specific fraud vector (take cash → void
+  the record) largely disappears — but it's replaced by **unattended-machine threats**: tailgating,
+  plate spoofing/obscuring, physical tampering with a box nobody is watching, and forced entry.
+  The [[append-only-event-chain]] + [[reconciliation]] controls still apply; the emphasis moves
+  from "catch the cashier" to "trust the automated record + detect tampering."
+- **Host-in-the-loop entry becomes mandatory, not optional.** With no person to hand over a ticket
+  or wave a car through, the machine must own the whole flow: detect arrival → issue ticket / read
+  plate → open. This is exactly why the [[access-controller-button-flow]] blocker matters and why
+  a controller whose input does **not** auto-fire the relay (see [[dingtian-relay]]) is required.
+- **Reliability / fail-state get more critical** ([[fail-state-safety]]). No operator to recover a
+  stuck barrier or a trapped car ⇒ watchdogs, **exit-fails-open**, and hardware manual override
+  stop being nice-to-haves. Unattended uptime is a hard requirement.
+- **Payment goes unmanned.** Pay-station / pay-on-foot or in-lane unmanned terminal rather than a
+  booth P2PE + cash drawer — sharpens [[open-questions]] #3 toward the unmanned option (PCI scope
+  still kept out of the app via a certified terminal).
+- **Identity leans on automation.** Plate recognition ([[lpr-camera]]) and permit reads become the
+  primary identity sources, since there's no one to issue/inspect a paper ticket by hand.
+
+## Near-term stance
+
+Build for the unmanned target but don't over-engineer ahead of it. Current concrete step: the
+**[[dingtian-relay]]** controller over **HTTP** (device pushes input events to the host; host
+commands relays) — see [[dingtian-vs-mqtt]] for why HTTP over a message bus for now.
+
+## Open
+
+Lane topology, payment subsystem, and reconciliation channel ([[open-questions]]) should all be
+(re)evaluated through the **unmanned** lens before procurement.

wiki/decisions/dingtian-vs-mqtt.md

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+---
+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.