Behavision: face recognition for retail, edge to head office

Five components that ship as one product:

- behavision/  the recognition engine. RTSP ingest, YuNet detection, IoU
               tracking, ArcFace embeddings, a FAISS/SQLite gallery, and a
               FastAPI dashboard. Identity is decided once per TRACK from an
               average of at least three embeddings, never per frame.
- agent/       the Go edge agent: supervises the engine, holds a durable
               spool, and drains it to MQTT. Nothing is acked before the
               broker confirms.
- desktop/     the shop PC application (Wails + React + tray).
- server/      the cloud API, MQTT consumer, reports and assistant.
- web/         platform.loyaly.ai, the head-office app, embedded in the
               server binary.

The gallery stores 512-float embeddings and timestamps - no images unless
`app.store_faces` is switched on. Those embeddings are biometric personal
data under GDPR and India's DPDP: template inversion reconstructs a
recognisable face from an ArcFace vector, so data/behavision.db is treated
as a biometric database and DELETE /api/visitors/{id} is a real erasure.

CLAUDE.md carries the reasoning behind every non-obvious decision here,
including the ones that were measured and the ones that were wrong first.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HViLj9gYNRtSr7YVZmW5sn

web/src/hooks.js

@@ -0,0 +1,42 @@
+import { useCallback, useEffect, useRef, useState } from 'react'
+
+// usePolled runs `fn` now and every `everyMs`, and is careful about the two
+// things every screen would otherwise get wrong on its own: overlapping
+// requests when the server is slower than the interval, and setting state
+// after the component has gone.
+export function usePolled(fn, everyMs, deps = []) {
+  const [data, setData] = useState(null)
+  const [error, setError] = useState(null)
+  const [loading, setLoading] = useState(true)
+  const inFlight = useRef(false)
+  const alive = useRef(true)
+
+  const run = useCallback(async () => {
+    if (inFlight.current) return
+    inFlight.current = true
+    try {
+      const out = await fn()
+      if (!alive.current) return
+      setData(out)
+      setError(null)
+    } catch (err) {
+      if (!alive.current) return
+      setError(err)
+    } finally {
+      inFlight.current = false
+      if (alive.current) setLoading(false)
+    }
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, deps)
+
+  useEffect(() => {
+    alive.current = true
+    run()
+    if (!everyMs) return () => { alive.current = false }
+    const t = setInterval(run, everyMs)
+    return () => { alive.current = false; clearInterval(t) }
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [run, everyMs])
+
+  return { data, error, loading, reload: run }
+}