feat: add git pull to update cached repositories from remote

- Replace dead FetchRemote with PullRepo (fetch + merge) in services/git.go
- Add POST /api/repos/:id/pull endpoint with DB size/last_used update
- Add '更新' button next to each cached repo in the index page

templates/pages/index.html

@@ -52,9 +52,15 @@
                                 <a href="/repo/{{.ID}}" class="text-sm font-medium text-indigo-600 hover:text-indigo-800">{{.URL}}</a>
                                 <p class="text-xs text-gray-400 mt-1">最后使用: {{.LastUsed}}</p>
                             </div>
-                            <button hx-delete="/api/repos/{{.ID}}" hx-confirm="确定删除此仓库缓存？"
-                                    hx-swap="none" hx-on::after-request="window.location.reload()"
-                                    class="text-xs text-red-500 hover:text-red-700">删除</button>
+                            <div class="flex gap-2">
+                                <button hx-post="/api/repos/{{.ID}}/pull" hx-swap="none"
+                                        hx-on::before-request="this.disabled=true;this.textContent='更新中...'"
+                                        hx-on::after-request="window.location.reload()"
+                                        class="text-xs text-indigo-500 hover:text-indigo-700">更新</button>
+                                <button hx-delete="/api/repos/{{.ID}}" hx-confirm="确定删除此仓库缓存？"
+                                        hx-swap="none" hx-on::after-request="window.location.reload()"
+                                        class="text-xs text-red-500 hover:text-red-700">删除</button>
+                            </div>
                         </div>
                         {{end}}
                     </div>