fix(security): apply audit remediations to dev (ported + dev-only)

Dev-branch audit (docs/_archive/security-audit-dev-2026-07-22.md) re-found the
master issues (absent on dev) plus new ones in dev-only code. Applied here;
full-node build + test suite green; the subtle fixes were adversarially re-verified.

Ported from the master remediation (adapted to dev's code):
- bootstrap: reject zip-slip / path-traversal archive members (isSafeArchivePath)
  before writing (S2-1). (dev already fail-closes on a missing checksum.)
- xmrig updater: fail closed when a signature is required but no key is pinned (F1-1).
- http_download.httpGetString: 16 MiB hard cap + MAXFILESIZE on the shared
  metadata/price fetch (F1-2 / caps the updater + exchange paths at one site).
- rpc_client: explicit SSL_VERIFYPEER/VERIFYHOST (F4-1) and a 256 MiB response
  cap in WriteCallback (F4-3).
- lite_connection_service: reject remote http:// lite servers, loopback only (L1-1).
  Loopback is matched by a strict dotted-decimal 127.0.0.0/8 check (not a
  startsWith("127.") prefix, which would wrongly accept 127.0.0.1.evil.com), with
  userinfo/fragment stripping.
- lite controller: propagate encrypt/decrypt save() failure instead of reporting
  success (F7-1).
- xmrig_manager: chmod(0600) the pool config before writing secrets (F5-2).
- app: clear the copied secret from the OS clipboard on shutdown (F3b-1).
- export_transactions: neutralize CSV/spreadsheet formula injection (F13-1).
- build pipeline: build-from-source lite backend + remove the self-attested
  CMake signature gate (F15-1); pinned+verified appimagetool (F15-3/4);
  verified Sapling params in setup.sh (F15-6); build.sh exits 0 on success.
  (F14-1 empty-quoted-arg and F8-2 NUL-termination were already fixed on dev.)

Dev-only findings:
- rpc_client.callRaw: scrub the raw buffer + parsed tree (templated scrubJsonSecrets
  for ordered_json) so console dumpprivkey/z_exportkey keys don't linger in freed
  heap (N1-1).
- seed_wallet_creator: wipe the exported mnemonic on the failure path so a discarded
  failed result never carries a live seed (W1-2).
- export_all_keys: write the plaintext key dump 0600 + atomically via
  writeFileAtomically (U1-2).
- chat_database: restrict chat_messages.sqlite and its WAL/SHM sidecars to owner-only (C3-1).

Not done (need a decision, documented in the report):
- Chat header metadata (cid/z/p) rides outside the AEAD (C1/C2) — binding it is a
  wire-protocol change requiring SilentDragonXLite interop review.
- Bootstrap lacks an offline-rooted signature (S2-2 residual) — needs signing infra.
- Console scrollback retains console-typed key-export output in plaintext (N1-1
  residual) — inherent to an echoing console; would need output redaction.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/util/http_download.cpp

@@ -19,10 +19,19 @@ namespace util {
 
 namespace {
 
+// Cap for in-memory text/JSON responses (release metadata, price/candle data). Far above any
+// legitimate body, but bounds memory if a hostile/MITM'd server streams an unbounded response.
+constexpr std::size_t kMaxMetadataBytes = 16u * 1024 * 1024;
+
 size_t writeStringCb(void* contents, size_t size, size_t nmemb, void* userp)
 {
-    static_cast<std::string*>(userp)->append(static_cast<char*>(contents), size * nmemb);
-    return size * nmemb;
+    auto* s = static_cast<std::string*>(userp);
+    const size_t n = size * nmemb;
+    // Hard cap: a chunked response omits Content-Length, so CURLOPT_MAXFILESIZE cannot catch it —
+    // aborting here (short count) makes curl fail the transfer instead of exhausting memory.
+    if (s->size() + n > kMaxMetadataBytes) return 0;
+    s->append(static_cast<char*>(contents), n);
+    return n;
 }
 
 size_t writeFileCb(void* contents, size_t size, size_t nmemb, void* userp)
@@ -52,6 +61,7 @@ std::string httpGetString(const std::string& url, const char* logTag)
     curl_easy_setopt(curl, CURLOPT_USERAGENT, "ObsidianDragon/1.0");
     curl_easy_setopt(curl, CURLOPT_TIMEOUT, 30L);
     curl_easy_setopt(curl, CURLOPT_CONNECTTIMEOUT, 15L);
+    curl_easy_setopt(curl, CURLOPT_MAXFILESIZE_LARGE, static_cast<curl_off_t>(kMaxMetadataBytes));  // reject oversized metadata
     curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, 1L);
     curl_easy_setopt(curl, CURLOPT_SSL_VERIFYHOST, 2L);
     const CURLcode res = curl_easy_perform(curl);