Add shareable search links and watched movie indicators

Share: "Copy Link" button generates a URL with ?s={history_id} that
loads the saved search results without auth. Recipients see the same
movie picks.

Watched: When viewing history results or shared searches, cards for
movies the logged-in user has since watched are dimmed with a green
"Watched" badge. Uses a new POST /api/library/watch-check endpoint.

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

app/static/index.html

@@ -109,10 +109,13 @@
             <div id="results" class="hidden">
                 <div id="results-grid" class="grid grid-cols-1 sm:grid-cols-2 lg:grid-cols-3 gap-6"></div>
                 <div id="results-meta" class="mt-6 text-center text-gray-500 text-sm"></div>
-                <div class="mt-4 text-center">
+                <div class="mt-4 flex items-center justify-center gap-3">
                     <button id="reroll-btn" class="px-6 py-2 border border-indigo-600/50 text-indigo-400 hover:bg-indigo-600/20 rounded-lg font-semibold transition-colors text-sm">
                         Show Me More
                     </button>
+                    <button id="share-btn" class="px-4 py-2 border border-gray-700 text-gray-500 hover:text-gray-300 hover:border-gray-500 rounded-lg transition-colors text-sm">
+                        Copy Link
+                    </button>
                 </div>
             </div>