Fix five P0/P1 security & correctness findings from the full-eval

- Arbitrary file write (P0): validate import keys in /api/library/import via
  a now-exported safeFilename(); a ../../ key is skipped, not written out of
  the scope dir.
- SSRF (P0): guard downloadPodcastAudio — reject non-HTTP(S) schemes, block
  IP-literal and DNS-resolved private/link-local/loopback/reserved/multicast
  and embedded-IPv4 IPv6 targets (closes DNS rebinding), cap + resolve redirects.
- ESM require (P1): top-level import of randomBytes in license-purchase.js
  (the inner require threw on the anon purchase-settle path).
- Concurrency lock (P1): skip the process-global free-tier slot in multi-mode
  so it no longer serializes every cloud tenant onto one job.
- X-Forwarded-For bypass (P1): set Express trust proxy from
  RECAP_TRUSTED_PROXY_HOPS (default 1); getClientIp now reads req.ip instead
  of a client-spoofable XFF entry.

Tests added for safeFilename, the SSRF guard, and getClientIp (119 pass).
Registry blockers deferred (ROADMAP); leaked-key history purge queued.

server/history.js

@@ -653,7 +653,7 @@ export function setupHistoryRoutes(app, { addToSkipList } = {}) {
 // Allow the same character set as scope components for session ids.
 // Belt-and-suspenders against ../../ in :id; ids generated by
 // saveToHistory always match.
-function safeFilename(s) {
+export function safeFilename(s) {
   if (typeof s !== "string" || !/^[A-Za-z0-9_-]+$/.test(s)) {
     throw new Error("invalid_session_id");
   }