Add a "needs review" flag with a sidebar filter

Links can be bookmarked for a later look via a toggle button on the
card (🏷️/🔖), shown with a yellow border while flagged. A new
sidebar section "Merkliste" links to /?review=1, filtering to just
the flagged links; the count updates live via the existing facets
query.

Since the project has no migration framework, add a minimal
startup check that ALTER TABLEs in the needs_review column for
existing SQLite databases (create_all only creates missing tables).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/templates/_link_card.html

@@ -1,4 +1,4 @@
-<div class="panel link-card" id="link-{{ link.id }}">
+<div class="panel link-card {% if link.needs_review %}needs-review{% endif %}" id="link-{{ link.id }}">
   <div class="card-head">
     <span class="card-avatar" style="--h: {{ link.url | hue }};">
       {{ (link.url | domain)[:1] | upper or '?' }}
@@ -14,6 +14,12 @@
     </div>
 
     <div class="card-actions">
+      <button class="ghost review-toggle {% if link.needs_review %}active{% endif %}"
+              style="padding:3px 7px; font-size:.8rem;"
+              title="{% if link.needs_review %}Nicht mehr zur Durchsicht{% else %}Zur Durchsicht markieren{% endif %}"
+              hx-post="/links/{{ link.id }}/toggle-review"
+              hx-target="#link-{{ link.id }}"
+              hx-swap="outerHTML">{% if link.needs_review %}🔖{% else %}🏷️{% endif %}</button>
       <button class="ghost" style="padding:3px 7px; font-size:.8rem;"
               title="Bearbeiten"
               hx-get="/links/{{ link.id }}/edit"