feat: 블로그 시즌 연동 — 홈페이지 세무 정보 섹션 시즌화

- TaxSeason / CurrentSeasonDto에 RelatedCategorySlug 추가
- TaxSeasonCalendar 각 시즌에 카테고리 슬러그 매핑
  (income-tax→income-tax, vat-1st/2nd→vat, 종부세→real-estate-tax 등)
- IBlogPostRepository.GetByCategorySlugAsync 추가
- BlogService.GetSeasonalPostsAsync: 시즌 관련 글 2개 우선 + 나머지 최신 글로 채움
- IndexModel: SeasonalPosts / RecentPosts 분리 로드
- Index.cshtml 블로그 섹션: 시즌 중 "이번 시즌 추천" 배지 + 시즌별 전체보기 버튼
- site.css: blog-card--seasonal, seasonal-blog-tag, btn-seasonal 스타일

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

TaxBaik.Web/wwwroot/css/site.css

@@ -641,3 +641,65 @@ img {
     white-space: nowrap;
     letter-spacing: 0.5px;
 }
+
+/* ===== 블로그 시즌 연동 ===== */
+.seasonal-blog-header {
+    display: flex;
+    justify-content: center;
+}
+.seasonal-blog-tag {
+    display: inline-block;
+    background: linear-gradient(135deg, #C62828 0%, #B71C1C 100%);
+    color: white;
+    font-size: 0.82rem;
+    font-weight: 700;
+    padding: 4px 16px;
+    border-radius: 20px;
+    letter-spacing: 0.5px;
+}
+
+.blog-card--seasonal {
+    border: 2px solid var(--color-primary) !important;
+    position: relative;
+    overflow: visible;
+}
+.blog-seasonal-ribbon {
+    position: absolute;
+    top: -12px;
+    left: 50%;
+    transform: translateX(-50%);
+    background: var(--color-primary);
+    color: white;
+    font-size: 0.75rem;
+    font-weight: 700;
+    padding: 3px 14px;
+    border-radius: 20px;
+    white-space: nowrap;
+    letter-spacing: 0.5px;
+    z-index: 1;
+}
+
+.bg-season-badge {
+    background-color: var(--color-primary) !important;
+    color: white !important;
+}
+
+.btn-seasonal {
+    background-color: var(--color-primary);
+    color: white;
+    border: none;
+}
+.btn-seasonal:hover {
+    background-color: var(--color-primary-dark);
+    color: white;
+}
+
+.btn-outline-seasonal {
+    border: 2px solid var(--color-primary);
+    color: var(--color-primary);
+    background: transparent;
+}
+.btn-outline-seasonal:hover {
+    background-color: var(--color-primary);
+    color: white;
+}