wiki: design the business layer (session, tariff, permit, vision, shift, ops)

Pivot from the hardware/integrity layer to the parking operation. All
wiki-only; no code yet. Core principle throughout: business entities are
projections over the signed append-only event log, never mutable tables.

New concepts: parking-session, tariff (composable/versioned, FX-ready),
shift (manned-only Z-report), capacity-occupancy, validation-discounts,
reporting-analytics, clock-integrity, ticket-encoding, anti-passback.
New entities: permit, opencv-anpr-service, blocklist.
Decisions: session-model, vision-service (host-side ANPR + vehicle
verification; scoped AGPL exception for the isolated service).

Updates: append-only-event-chain (new event types + vision witness),
local-jwt-auth (drop 8h expiry -> until logout; code change pending),
lpr-camera (host-side recognition supersedes edge-AI), standing-decisions
(AGPL exception), open-questions (+FX, +pay-station money corners, backup).

Deferred + flagged: intercom/help-call, receipts/refunds/change, FX engine,
lane topology (#1).

wiki/concepts/anti-passback.md

@@ -0,0 +1,57 @@
+---
+type: concept
+tags: [parking, domain, business, anti-fraud, access-control]
+sources: []
+updated: 2026-06-15
+status: open
+---
+
+# Anti-Passback
+
+Stop one credential/ticket from getting **two cars in** without an exit between — the classic
+"pass the card/ticket back over the fence" abuse. A control on the entry validation, leaning on the
+session projection.
+
+## The rule
+
+An identity (ticket id, [[permit]] credential, or plate) **must not enter while it already has an
+OPEN [[parking-session|session]].** At entry:
+
+```
+identify vehicle → is there already an OPEN session for this id?
+   no  → proceed (mint vehicle_entry, open)
+   yes → passback violation → refuse or flag (see policy)
+```
+
+This is a **fold over the signed [[append-only-event-chain]]** ("does an entry for this id exist
+with no matching exit?") — not a mutable in/out flag that could be edited. Same projection that
+powers [[capacity-occupancy]] and [[permit]] `maxConcurrent`.
+
+## Interaction with the limits already designed
+
+- **Transient ticket** — a single ticket id is inherently one session; a second entry on the same
+  id is always a violation (or a re-print/duplication attempt).
+- **Permit** — passback is the *per-car* case of the permit's `maxConcurrent` ([[permit]]): a
+  multi-car permit legitimately has several open sessions, but **the same car/credential** entering
+  twice is still a violation. So enforce per-identity, *under* the permit's concurrency allowance.
+
+## Policy (operator choice)
+
+- **Hard** — refuse the second entry (strict; risks stranding a legitimate car after a *missed
+  exit*, which is common — tailgated out, sensor missed).
+- **Soft** — allow but **flag an `anomaly`** (the type exists) for review. Safer against
+  false-positives from missed exits, consistent with the append-only "record + flag, don't block"
+  ethos elsewhere.
+- Likely **soft by default**, hard as an opt-in for high-control sites.
+
+## Honest limits
+
+- Depends on **reliable exit detection** — if exits are routinely missed (no exit loop/plate read),
+  passback produces false positives; tune to the site's exit fidelity.
+- A spoofed/duplicated ticket QR is caught here (same id already open) — complements
+  [[ticket-encoding]]'s opaque-id requirement.
+
+## Open
+
+- Default policy (soft/hard) and per-site override.
+- Grace for legitimate quick re-entry vs. the missed-exit false-positive.