feat(dashboard): add corpus dashboard at /dashboard/

Full private corpus dashboard for tools.dobetternorge.no users — each SSO
account gets an auto-provisioned CaveauAI tenant (clients row, corpus) on
first visit. Includes upload (file/paste/URL), RAG chat with SSE streaming
and citation chips, document CRUD, FalkorDB graph relations tab, and
improved save-from-tool flow with tag/preview support.

- dashboard/{index,documents,document,upload,chat,settings}.php
- api/dashboard/{corpus-init,documents,upload,ingest-status,chat-stream,
  save-from-tool,graph}.php
- includes/{CorpusProvision,layout_dashboard,layout_dashboard_footer}.php
- assets/css/dashboard.css  assets/js/corpus-save.js (routing upgrade)
- includes/{bootstrap,layout}.php extended for dashboard provisioning

Migration 141 (clients.dbn_sso_uid + import_method enum) applied on chloe.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

includes/layout.php

@@ -67,6 +67,7 @@ window.DBN_FREE_TIER_BALANCE = <?= $layoutFreeTierBalance ?>;
                 <a href="<?= htmlspecialchars($langPath . '?lang=' . $langCode) ?>" class="<?= $langCode === $uiLang ? 'is-active' : '' ?>"><?= htmlspecialchars(dbnToolsLanguageLabel($langCode)) ?></a>
 <?php endforeach; ?>
             </nav>
+            <a href="/dashboard/" class="secondary-button" style="text-decoration:none;">📚 Min korpus</a>
             <span id="healthPill" class="status-pill"><?= htmlspecialchars(dbnToolsT('session_active', $uiLang)) ?></span>
             <button id="healthButton" class="secondary-button" type="button"><?= htmlspecialchars(dbnToolsT('health', $uiLang)) ?></button>
         </div>