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/styles.css

@@ -50,3 +50,23 @@
     border-color: #22c55e;
     color: #86efac;
 }
+
+.movie-card.watched {
+    opacity: 0.45;
+}
+.movie-card.watched::after {
+    content: 'Watched';
+    position: absolute;
+    top: 12px;
+    right: 12px;
+    background: rgba(34, 197, 94, 0.8);
+    color: white;
+    font-size: 0.7rem;
+    font-weight: 600;
+    padding: 2px 8px;
+    border-radius: 9999px;
+    z-index: 10;
+}
+.movie-card.watched {
+    position: relative;
+}