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>

api/dashboard/corpus-init.php

@@ -0,0 +1,38 @@
+<?php
+/**
+ * GET /api/dashboard/corpus-init.php
+ *
+ * Idempotent: ensures the current session has a CaveauAI client tenant +
+ * default corpus, lazy-creating both on first hit. Safe to call on every
+ * dashboard page load (results are session-cached).
+ *
+ * Response:
+ *   {
+ *     "ok": true,
+ *     "client_id": 102,
+ *     "client_user_id": 257,
+ *     "corpus_id": 18,
+ *     "created": false
+ *   }
+ */
+
+declare(strict_types=1);
+
+require_once dirname(__DIR__, 2) . '/includes/bootstrap.php';
+
+dbnToolsRequireMethod('GET');
+dbnToolsRequireAuth();
+
+try {
+    $tenant = dbnToolsEnsureDashboardTenant();
+} catch (DbnToolsHttpException $e) {
+    dbnToolsError($e->getMessage(), $e->status, $e->errorCode, $e->extra);
+}
+
+dbnToolsRespond([
+    'ok'             => true,
+    'client_id'      => (int)$tenant['client_id'],
+    'client_user_id' => (int)$tenant['client_user_id'],
+    'corpus_id'      => (int)$tenant['corpus_id'],
+    'created'        => (bool)($tenant['created'] ?? false),
+]);