Rongta 80mm printer: driver, role-based failover, live status monitoring

Add the rongta PrinterDevice driver (ESC/POS over raw TCP 9100) and the
device-agnostic pieces around it:

- Roles + failover: each printer declares a role (entry-dispenser/booth-
  receipt) and failoverRank; printer-routing.ts picks the best healthy printer
  and falls back outside->booth for entry tickets (never the reverse).
- Live status: MonitorableDevice.readStatus()/PrinterStatus capability. The
  Rongta driver scrapes the device's own /prn_stat.htm (Cover/Cutter/Paper
  End/Near End/Off-Line) rather than hand-decoding DLE EOT, whose reply bytes
  on this clone don't match the canonical ESC/POS bit layout (verified on
  hardware) -- avoids a false-healthy. Maps to ready/degraded/offline, fail
  safe on an unreachable or unexpected page.
- Server PrinterMonitor polls enabled printers (PRINTER_POLL_MS, default 5s),
  caches latest, emits "printer-status" on change. Exposed via
  GET /api/printers/status and an SSE stream for the booth UI.

Verified against 10.0.10.6: ready when healthy, offline when unreachable
(no throw), bus emits on change and suppresses unchanged reads.

Wiki: new rongta-printer entity, printer-roles-failover and
printer-status-monitoring concepts; BOM/index/log updated.

wiki/concepts/printer-roles-failover.md

@@ -0,0 +1,53 @@
+---
+type: concept
+tags: [parking, printer, device, reliability]
+sources: []
+updated: 2026-06-14
+---
+
+# Printer roles & failover
+
+A lane runs **more than one printer**, and the system knows each one's job so it can fail over
+automatically. This is a reliability decision, not a threat-model one: an entry ticket must
+still print when the outside dispenser jams or drops off the network.
+
+## Roles
+
+Each printer instance (a `lane_devices` row, category `printer`) declares a **role** in its
+config:
+
+- **`entry-dispenser`** — outside, at the lane. Prints the entry ticket the driver takes.
+- **`booth-receipt`** — inside the booth. Prints receipts at exit/payment, AND serves as the
+  **backup** for entry tickets.
+
+It also declares a **`failoverRank`** (higher = preferred within a role) to order multiple
+printers of the same role deterministically (ties broken by id).
+
+## Failover rule (asymmetric, on purpose)
+
+For an **entry ticket** (`wantRole = entry-dispenser`): try the entry dispensers (best rank
+first), then fall back to the **booth printer**. So a driver still gets a ticket when the
+outside unit is offline — the operator hands it over from the booth.
+
+The reverse is **deliberately not** done: a **receipt** never prints on the outside dispenser.
+Receipts are a booth-only job; an entry dispenser falling back to print receipts makes no
+physical sense.
+
+## Where the logic lives
+
+- The driver (`rongta`) is **role-agnostic** — role/rank are just config; the transport doesn't
+  care. Keeps [[device-adapter-pattern|adapters]] swappable.
+- Selection is pure logic in `packages/devices/printer-routing.ts`: `orderForRole()` ranks
+  candidates; `printWithFailover()` attempts the print down the list and throws
+  `NoPrinterAvailableError` only when every candidate fails.
+- It **attempts the print directly** rather than racing a `healthCheck` first — the print is
+  the real reachability test, and a health probe that passes can still be followed by a failed
+  print.
+
+## Open: the all-printers-down policy
+
+When `printWithFailover` exhausts every candidate, what should entry do — raise the barrier
+with no paper ticket (the plate/[[lpr-camera]] is the independent record), or hold? That policy
+belongs to the **entry flow** ([[device-input-flow]], [[fail-state-safety]]), not the printer
+layer, and is **not yet decided**. The signed event ([[append-only-event-chain]]) is created
+regardless of whether paper prints.