feat: document & audio corpus picker for all tools

- Add "Select from My Docs" button to all text tool forms; free-tier
  users see an upgrade modal, paid (CaveauAI) users get a searchable
  multi-select modal backed by /api/dashboard/documents.php
- Add "Select from My Audio" picker on Transcribe with single-select
  and a "Save to My Audio" button for persisting uploaded clips
- New PHP helpers in bootstrap.php: dbnToolsFetchDocChunks,
  dbnToolsClientIdFromSession, dbnToolsInjectDocContent
- timeline, ask, redact APIs prepend selected document content
  (fetched from client_chunks SQL) before the textarea text
- api/dashboard/audio-upload.php stores audio files on server and
  creates a client_documents row with source_type='audio'
- api/transcribe.php falls back to stored audio via audio_doc_id POST
  field when no file is uploaded
- api/dashboard/documents.php supports ?source_type= filter
- tools.js: doc_ids added to JSON payload; stored-audio transcribe path
- New assets/css/doc-picker.css, assets/js/doc-picker.js
- SQL migration: scripts/sql/audio_docs_column.sql

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

api/dashboard/documents.php

@@ -79,6 +79,12 @@ function respondList(PDO $db, int $clientId): void
         $where[]  = 'category = ?';
         $params[] = $category;
     }
+    $sourceType = trim((string)($_GET['source_type'] ?? ''));
+    $allowedSourceTypes = ['text', 'audio', 'url', 'tool-output', 'upload'];
+    if ($sourceType !== '' && in_array($sourceType, $allowedSourceTypes, true)) {
+        $where[]  = 'source_type = ?';
+        $params[] = $sourceType;
+    }
 
     $whereSql = 'WHERE ' . implode(' AND ', $where);