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/test/anon-trial.test.js

@@ -0,0 +1,37 @@
+// Tests for server/anon-trial.js — focused on getClientIp, which underpins
+// the per-IP trial cap. The DB-backed minting paths need a multi-mode SQLite
+// handle and are exercised by integration tests; here we lock down that the
+// client IP is taken from Express's trust-proxy-resolved req.ip, never from a
+// raw client-supplied X-Forwarded-For header.
+
+import { test, describe } from "node:test";
+import { strict as assert } from "node:assert";
+import { getClientIp } from "../anon-trial.js";
+
+describe("getClientIp", () => {
+  test("uses req.ip (Express's trust-proxy-resolved client address)", () => {
+    assert.equal(getClientIp({ ip: "203.0.113.7" }), "203.0.113.7");
+  });
+
+  test("strips the IPv4-mapped IPv6 prefix", () => {
+    assert.equal(getClientIp({ ip: "::ffff:203.0.113.7" }), "203.0.113.7");
+  });
+
+  test("falls back to the socket address when req.ip is absent", () => {
+    assert.equal(
+      getClientIp({ socket: { remoteAddress: "::ffff:198.51.100.9" } }),
+      "198.51.100.9",
+    );
+  });
+
+  test("does NOT trust a raw client-supplied X-Forwarded-For header", () => {
+    // Express, not getClientIp, decides the client IP from trust proxy. A
+    // header Express hasn't blessed must be ignored — so with no req.ip we
+    // fall through to the socket address, never the spoofed header value.
+    const spoofed = {
+      headers: { "x-forwarded-for": "1.2.3.4" },
+      socket: { remoteAddress: "203.0.113.7" },
+    };
+    assert.equal(getClientIp(spoofed), "203.0.113.7");
+  });
+});