Add CPR cross-referencing (related documents)

Clicking any flagged card that contains CPR hits now shows a "Related documents" section in the preview panel,
  listing other items from the same scan session that share at least one CPR number. Items are ordered by number of
  shared CPRs; clicking any entry opens it in the preview panel. Works in both live mode and scan history mode.

  Implementation
  - GDPRDb.get_related_items() — SQL self-join on the existing cpr_index table using the same symmetric 300 s session
  window as get_session_items. No new data collection needed.
  - GET /api/db/related/<item_id>?ref=N — new endpoint in routes/database.py, consistent with the ?ref convention used
   by /api/db/flagged.
  - #previewRelated div injected between the metadata block and disposition row in the preview panel.
  - _loadRelated(f) in results.js fetches and renders the list; window._openRelated() resolves items from the live
  grid or falls back to the API response for history-mode items.

  Also
  - Added keyword/FTS5 search as a deferred idea in SUGGESTIONS.md
  - Updated CHANGELOG.md, README.md, and CLAUDE.md

templates/index.html

@@ -478,6 +478,8 @@ document.addEventListener('DOMContentLoaded', applyI18n);
               <iframe id="previewFrame" sandbox="allow-scripts allow-same-origin allow-forms allow-popups" style="display:none"></iframe>
             </div>
             <div class="preview-meta" id="previewMeta"></div>
+            <!-- Related documents -->
+            <div id="previewRelated" style="display:none;padding:8px 14px 4px;border-top:1px solid var(--border)"></div>
             <!-- Disposition widget (#6) -->
             <div class="disposition-row" id="dispositionRow" style="display:none">
               <span class="disposition-label" data-i18n="m365_disposition_label">Disposition</span>