feat: free-tier credit system + Syttende Mai access for Google users

- FreeTier.php: credit check/deduct/reset engine with hourly rate limit
- bootstrap.php: dbnmDb() singleton, dbnToolsIsFreeTier(), credit gate helpers
- index.php: store tier=free|approved in session from SSO JWT
- All 7 API endpoints: credit gate (402/429) + X-Credits-Remaining header
- layout.php: credit meta tag, JS balance var, Syttende Mai banner (05-17 only)
- tools.js: credit badge in topbar, 402 modal, 429 toast, dbnUpdateCredits()
- barnevernet.js + deep-research.js: wire 402/429 handling for NDJSON streams
- tools.css: styles for credit badge, no-credits modal, rate-limit toast

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/transcribe.php

@@ -5,6 +5,9 @@ require_once __DIR__ . '/../includes/LegalTools.php';
 
 dbnToolsRequireMethod('POST');
 dbnToolsRequireAuth();
+$ftUid = dbnToolsFreeTierCheck('transcribe');
+$ftRemaining = dbnToolsFreeTierDeduct($ftUid, 'transcribe');
+if ($ftRemaining >= 0) { header('X-Credits-Remaining: ' . $ftRemaining); }
 
 set_time_limit(0);
 ignore_user_abort(true);