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

main.go

@@ -114,6 +114,7 @@ func main() {
 	r.POST("/api/repos", authMw, repos.CloneRepo)
 	r.DELETE("/api/repos/:id", authMw, repos.DeleteRepo)
 	r.POST("/api/repos/:id/cleanup", authMw, repos.CleanupRepos)
+	r.POST("/api/repos/:id/pull", authMw, repos.PullRepo)
 	r.GET("/api/repos/:id/graph", authMw, repos.GetGraph)
 	r.GET("/api/repos/:id/refs", authMw, repos.GetRefs)
 	r.GET("/api/repos/:id/commits", authMw, repos.GetCommits)