feat: 分类系统 + 标签系统 — Phase 4 完成

分类：Model/Repo/Service/Controller + 管理后台树形页面 + 首页导航栏
标签：Model/Repo/Service/Controller + /tags/{slug} 落地页 + 写文章页输入
Post 加 CategoryID，Create/Update 支持分类和标签
SQL 迁移含 categories/tags/post_tags 表及预设未分类

docs/migrations/004_categories_tags.sql

@@ -0,0 +1,32 @@
+-- 分类表（两级树形）
+CREATE TABLE IF NOT EXISTS categories (
+    id          SERIAL PRIMARY KEY,
+    name        VARCHAR(50)  NOT NULL,
+    slug        VARCHAR(50)  NOT NULL UNIQUE,
+    description VARCHAR(200) DEFAULT '',
+    parent_id   INT          REFERENCES categories(id) ON DELETE SET NULL,
+    sort        INT          DEFAULT 0,
+    created_at  TIMESTAMP    DEFAULT NOW(),
+    updated_at  TIMESTAMP    DEFAULT NOW()
+);
+
+-- 预设「未分类」
+INSERT INTO categories (id, name, slug) VALUES (1, '未分类', 'uncategorized') ON CONFLICT DO NOTHING;
+
+-- 标签表
+CREATE TABLE IF NOT EXISTS tags (
+    id          SERIAL PRIMARY KEY,
+    name        VARCHAR(50)  NOT NULL,
+    slug        VARCHAR(50)  NOT NULL UNIQUE,
+    created_at  TIMESTAMP    DEFAULT NOW()
+);
+
+-- 文章-标签关联表
+CREATE TABLE IF NOT EXISTS post_tags (
+    post_id     INT NOT NULL REFERENCES posts(id) ON DELETE CASCADE,
+    tag_id      INT NOT NULL REFERENCES tags(id) ON DELETE CASCADE,
+    PRIMARY KEY (post_id, tag_id)
+);
+
+-- 文章加分类外键
+ALTER TABLE posts ADD COLUMN IF NOT EXISTS category_id INT REFERENCES categories(id) ON DELETE SET NULL;