db: add performance indexes on posts, tags, sessions

migrations/002_indexes.sql

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+-- 按 slug 查询文章（文章详情页）
+CREATE INDEX IF NOT EXISTS idx_posts_slug ON posts(slug) WHERE deleted_at IS NULL;
+
+-- 按状态和时间查询（文章列表、归档）
+CREATE INDEX IF NOT EXISTS idx_posts_status_published ON posts(status, deleted_at, published_at DESC);
+
+-- 标签名称查询
+CREATE INDEX IF NOT EXISTS idx_tags_name ON tags(name);
+
+-- 文章标签关联查询
+CREATE INDEX IF NOT EXISTS idx_post_tags_post_id ON post_tags(post_id);
+CREATE INDEX IF NOT EXISTS idx_post_tags_tag_id ON post_tags(tag_id);
+
+-- 用户会话查询
+CREATE INDEX IF NOT EXISTS idx_sessions_user_id ON sessions(user_id);