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

services/git.go

@@ -441,19 +441,51 @@ func GetBranchCommits(repoPath, branchName string, maxCommits int) ([]CommitInfo
 	return GetCommitLog(repo, branchName, maxCommits)
 }
 
-// FetchRemote fetches latest changes for a repo.
-func FetchRemote(repoPath string) error {
+// PullRepo fetches and merges latest changes from origin into the current branch.
+func PullRepo(repoPath string) (*CloneResult, error) {
 	repo, err := OpenRepo(repoPath)
 	if err != nil {
-		return err
+		return nil, err
 	}
-remote, err := repo.Remote("origin")
+
+	w, err := repo.Worktree()
 	if err != nil {
-		return err
+		return nil, fmt.Errorf("worktree: %w", err)
 	}
-	return remote.Fetch(&git.FetchOptions{
-		Force: true,
-	})
+
+	err = w.Pull(&git.PullOptions{})
+	if err != nil && err != git.NoErrAlreadyUpToDate {
+		return nil, fmt.Errorf("git pull: %w", err)
+	}
+
+	result := &CloneResult{RepoPath: repoPath}
+
+	iter, err := repo.CommitObjects()
+	if err == nil {
+		_ = iter.ForEach(func(c *object.Commit) error {
+			result.CommitNum++
+			return nil
+		})
+	}
+
+	branches, err := repo.Branches()
+	if err == nil {
+		_ = branches.ForEach(func(ref *plumbing.Reference) error {
+			result.Branches = append(result.Branches, ref.Name().Short())
+			return nil
+		})
+	}
+
+	tags, err := repo.Tags()
+	if err == nil {
+		_ = tags.ForEach(func(ref *plumbing.Reference) error {
+			result.Tags = append(result.Tags, ref.Name().Short())
+			return nil
+		})
+	}
+
+	result.SizeBytes = dirSize(repoPath)
+	return result, nil
 }
 
 // dirSize returns the total size of files in a directory.