Add QR code tracker to admin panel

Each QR code gets a /qr/<slug> redirect that logs scans (IP, user agent,
timestamp) to disk. The admin QR Codes view lets you add, edit, enable/disable,
and delete codes, with per-code scan history inline.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/data.js

@@ -20,6 +20,7 @@ import {
   STUDY_COMMENTS_FILE,
   STUDY_CERTIFICATES_FILE,
   EPISODE_SCRIPTS_FILE,
+  QR_CODES_FILE,
   REPLY_TEMPLATES_FILE,
   REPLY_HISTORY_FILE,
   PODCAST_CHECKLIST_FILE,
@@ -1216,3 +1217,29 @@ export function sanitizeStudyCommunityPosts(value) {
     })
     .filter(Boolean)
 }
+
+// ── QR Codes ───────────────────────────────────────────────────────────────
+
+export function queueQrCodesWrite() {
+  state.qrCodesWritePromise = state.qrCodesWritePromise
+    .then(async () => {
+      await mkdir(DATA_DIR, { recursive: true })
+      await writeFile(QR_CODES_FILE, JSON.stringify({ codes: state.qrCodes, scans: state.qrScans }, null, 2), 'utf8')
+    })
+    .catch(err => {
+      console.error('[qr-codes] failed to write:', err)
+    })
+}
+
+export function loadQrCodesFromDisk() {
+  return readFile(QR_CODES_FILE, 'utf8')
+    .then(raw => {
+      const parsed = JSON.parse(raw)
+      state.qrCodes = Array.isArray(parsed?.codes) ? parsed.codes : []
+      state.qrScans = Array.isArray(parsed?.scans) ? parsed.scans : []
+    })
+    .catch(() => {
+      state.qrCodes = []
+      state.qrScans = []
+    })
+}