Security hardening: rate limiting, atomic locks, origin check, honest docs

API / Security:
- Add api/_helpers.php: shared send_security_headers(), verify_origin(),
  get_hmac_secret(), check_rate_limit(), read_json_locked(), write_json_locked()
- shorten.php: remove Access-Control-Allow-Origin:*, restrict to same-origin,
  rate-limit 20 req/h per IP, atomic JSON read+lock, HMAC secret from file
- verify.php: rate-limit GET (30/min) and POST (10/h) per IP, atomic lock,
  prevent overwriting existing proofs, origin check on POST
- node.php: fix rate limit from 1000 to 60 req/min, add security headers,
  origin check
- check-short.php: add security headers, re-derive signature server-side
- s.php: use file-based HMAC secret via get_hmac_secret(), hash_equals()
  for timing-safe comparison

Service Worker:
- sw.js: navigation requests (mode=navigate) never served from cache;
  network-first with offline fallback to prevent stale invoice state

Documentation (honest claims):
- README: tagline "No backend" -> "No tracking"; new Architecture table
  listing exactly what server sees for each feature; Security Model section
- index.html: meta description and footer updated from "No Backend" to
  "Minimal Backend"
- i18n.js footer: already updated in previous commit

api/check-short.php

@@ -1,15 +1,17 @@
 <?php
+<?php
+require_once __DIR__ . '/_helpers.php';
+
 /**
  * Short URL Integrity Verification API
- * GET: Return the hash and HMAC signature for client-side verification
- * 
- * Security: Allows client-side verification that the hash has not been
- * tampered with by the server. The signature is verified using the 
- * hostname as part of the secret HMAC key.
+ * GET: Return the hash and HMAC signature for client-side verification.
+ *
+ * Security: Allows client-side detection of server-side tampering.
+ * The HMAC secret is stored in data/secret.key (auto-generated on first run).
  */
 
 header('Content-Type: application/json');
-header('Access-Control-Allow-Origin: *');
+send_security_headers();
 
 if ($_SERVER['REQUEST_METHOD'] !== 'GET') {
     http_response_code(405);
@@ -42,9 +44,11 @@ $data = $urls[$code];
 $hash = is_array($data) ? $data['h'] : $data;
 $signature = is_array($data) ? $data['s'] : null;
 
-// Return hash and signature for client-side verification
+// Re-derive expected signature so client can verify
+$expected = $signature ? hash_hmac('sha256', $hash, get_hmac_secret()) : null;
+
 echo json_encode([
     'code' => $code,
     'hash' => $hash,
-    'signature' => $signature
+    'signature' => $expected
 ]);