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>

includes/tool_form.php

@@ -72,6 +72,15 @@
                         <p class="alias-hint">Replace a name with a bracketed alias, e.g. &ldquo;David Jr&rdquo; &rarr; [Junior]</p>
                     </div>
 
+                    <div id="docPickerSection" class="doc-picker-section">
+                        <button type="button" id="docPickerBtn" class="doc-picker-btn" aria-haspopup="dialog">
+                            <svg class="doc-picker-btn__icon" width="16" height="16" viewBox="0 0 16 16" fill="none" aria-hidden="true"><path d="M3 2h7l3 3v9H3V2z" stroke="currentColor" stroke-width="1.4" stroke-linejoin="round"/><path d="M10 2v3h3" stroke="currentColor" stroke-width="1.4" stroke-linejoin="round"/><path d="M5 7h6M5 9.5h4" stroke="currentColor" stroke-width="1.2" stroke-linecap="round"/></svg>
+                            <span>Select from My Docs</span>
+                        </button>
+                        <div id="docPickerChips" class="doc-picker-chips" aria-label="Selected documents"></div>
+                        <input type="hidden" id="docPickerIds" name="doc_ids" value="">
+                    </div>
+
                     <label class="input-label" for="toolInput" id="inputLabel">Question</label>
                     <textarea id="toolInput" name="toolInput" rows="10" required></textarea>