Fix 11 bugs: restore crash, draft leak, email failures, memory leaks

Critical fixes:
- sanitizeLoadedHitStats/VisitorStats: restore full state shape so a
  snapshot restore no longer crashes hit-counting middleware (missing
  byPathReal, byPathBot, byDayReal, byDayBot, botReasons, ipHashIndex)
- /questions/share/🆔 read state.questions only, not draft questions
- inbound-email: validate date with Number.isFinite before toISOString
- study-reminders: wrap each send in try/catch so one failure doesn't
  block remaining users; persist sent-markers after each success

Security:
- getClientIp: use req.ip (trust-proxy-resolved) instead of raw
  x-forwarded-for header to prevent IP spoofing
- env-snapshot.env: delete immediately after backup tar stream ends
  so secrets don't linger on disk between exports

Correctness / UX:
- contact form: email failures no longer 500 the user after the
  submission is already saved; log and fall through instead
- study-account profile: cap data URI avatar at 6 MB
- admin enrollment PATCH: validate slug against study catalog
- signup: return 503 at MAX_STUDY_USERS instead of silently dropping
  oldest accounts

Memory leaks:
- contactHits, downloadHits Maps: prune stale entries at 5000 entries
- resendEmailSubmissionIndex: trim to 2000 entries (oldest first)

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

server/routes/downloads.js

@@ -28,6 +28,11 @@ function studyDownloadRateLimit(req, res, next) {
   if (now - entry.start > windowMs) { entry.count = 0; entry.start = now }
   entry.count += 1
   downloadHits.set(ip, entry)
+  // Prune stale entries to prevent unbounded growth
+  if (downloadHits.size > 5000) {
+    const cutoff = now - windowMs
+    for (const [k, v] of downloadHits) { if (v.start < cutoff) downloadHits.delete(k) }
+  }
   if (entry.count > 10) {
     res.status(429).json({ message: 'Too many download requests. Please wait a few minutes.' })
     return