chore: initial commit

internal/db/schema/post_category.sql

@@ -0,0 +1,9 @@
+CREATE TABLE post_category
+(
+    post_id INT NOT NULL,
+    category_id INT NOT NULL,
+    PRIMARY KEY (post_id, category_id)
+);
+
+COMMENT ON COLUMN post_category.post_id IS '文章ID';
+COMMENT ON COLUMN post_category.category_id IS '分类ID';