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>

index.php

@@ -34,6 +34,7 @@ if (isset($_GET['sso']) && !dbnToolsIsAuthenticated()) {
             $_SESSION['dbn_tools_user_id']          = (int)$tokenData['uid'];
             $_SESSION['dbn_tools_user_email']       = (string)$tokenData['email'];
             $_SESSION['dbn_tools_user_role']        = 'sso';
+            $_SESSION['dbn_tools_tier']             = (string)($tokenData['tier'] ?? 'free');
             header('Location: ' . $returnPath);
             exit;
         }