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/study-auth.js

@@ -81,6 +81,11 @@ export function register(app) {
       return
     }
 
+    if (state.studyUsers.length >= MAX_STUDY_USERS) {
+      res.status(503).json({ message: 'Account registration is temporarily unavailable. Please try again later.' })
+      return
+    }
+
     const now = new Date().toISOString()
     const user = {
       id: randomUUID(),
@@ -97,9 +102,6 @@ export function register(app) {
     }
 
     state.studyUsers.push(user)
-    if (state.studyUsers.length > MAX_STUDY_USERS) {
-      state.studyUsers = state.studyUsers.slice(state.studyUsers.length - MAX_STUDY_USERS)
-    }
     queueStudyUsersWrite()
 
     if (subscribe) {