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/admin-assets.js

@@ -14,6 +14,8 @@ import {
 import {
   hashStudyPassword,
   getStudyCatalog,
+  normalizeStudySlug,
+  isEnrollableStudySlug,
 } from '../study-helpers.js'
 
 export function register(app) {
@@ -157,7 +159,10 @@ export function register(app) {
     }
 
     if (typeof addEnrollment === 'string' && addEnrollment.trim()) {
-      const slug = addEnrollment.trim()
+      const slug = normalizeStudySlug(addEnrollment)
+      if (!slug || !isEnrollableStudySlug(slug)) {
+        res.status(400).json({ message: `Unknown study slug: ${addEnrollment.trim()}` }); return
+      }
       if (!Array.isArray(user.enrolledStudySlugs)) user.enrolledStudySlugs = []
       if (!user.enrolledStudySlugs.includes(slug)) user.enrolledStudySlugs.push(slug)
     }