feat(ai-cases): 성공 사례 카드에 PDF 첫 슬라이드 썸네일·그라데이션 개선

- 목록에서 슬라이드 워밍(최대 24건) 후 coverImageUrl 전달
- 카드 상단 풀너비 이미지+스크림, 무이미지 시 다층 그라데이션

Made-with: Cursor

public/styles.css

@@ -428,33 +428,93 @@ button {
 
 .success-story-link {
   display: grid;
-  gap: 8px;
+  gap: 0;
   text-decoration: none;
   color: inherit;
+  padding: 0;
+}
+
+.success-story-link-body {
+  display: grid;
+  gap: 8px;
   padding: 12px 14px 14px;
 }
 
 .success-thumb {
-  border-radius: 10px;
-  min-height: 72px;
-  padding: 12px 14px;
-  background: linear-gradient(135deg, #0f766e, #14b8a6);
+  position: relative;
+  overflow: hidden;
+  border-radius: 12px 12px 0 0;
+  min-height: 100px;
   color: #fff;
+}
+
+/* PDF 첫 장(또는 슬라이드) 썸네일 */
+.success-thumb--cover {
+  min-height: 120px;
+  aspect-ratio: 16 / 9;
+  max-height: 200px;
+}
+
+.success-thumb-media {
+  position: absolute;
+  inset: 0;
+}
+
+.success-thumb-media img {
+  width: 100%;
+  height: 100%;
+  object-fit: cover;
+  display: block;
+}
+
+.success-thumb-scrim {
+  position: absolute;
+  inset: 0;
+  background: linear-gradient(
+    to top,
+    rgba(15, 23, 42, 0.88) 0%,
+    rgba(15, 23, 42, 0.45) 45%,
+    rgba(15, 23, 42, 0.2) 100%
+  );
+  pointer-events: none;
+}
+
+.success-thumb-inner {
+  position: relative;
+  z-index: 1;
+  padding: 12px 14px;
   display: flex;
   flex-direction: column;
-  justify-content: center;
+  justify-content: flex-end;
   gap: 4px;
+  min-height: 100%;
+  box-sizing: border-box;
+}
+
+/* 슬라이드 없음: 은은한 그라데이션 + 하이라이트 */
+.success-thumb--gradient {
+  background:
+    radial-gradient(120% 80% at 100% 0%, rgba(45, 212, 191, 0.45), transparent 55%),
+    radial-gradient(90% 70% at 0% 100%, rgba(14, 116, 144, 0.5), transparent 50%),
+    linear-gradient(135deg, #0c4a6e 0%, #0e7490 42%, #0d9488 78%, #14b8a6 100%);
+  box-shadow: inset 0 1px 0 rgba(255, 255, 255, 0.15);
+}
+
+.success-thumb--gradient .success-thumb-inner {
+  justify-content: center;
 }
 
 .success-thumb-icon {
   font-size: 22px;
   opacity: 0.95;
+  text-shadow: 0 1px 2px rgba(0, 0, 0, 0.25);
 }
 
 .success-thumb-kicker {
   font-size: 12px;
   font-weight: 600;
   letter-spacing: 0.02em;
+  text-shadow: 0 1px 3px rgba(0, 0, 0, 0.35);
 }
 
 .success-badge {