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

handlers/repos.go

@@ -140,6 +140,41 @@ func (h *ReposHandler) CleanupRepos(c *gin.Context) {
 	c.JSON(http.StatusOK, result)
 }
 
+// PullRepo handles POST /api/repos/:id/pull — fetches and merges latest changes.
+func (h *ReposHandler) PullRepo(c *gin.Context) {
+	user := GetCurrentUser(c)
+	if user == nil {
+		c.JSON(http.StatusUnauthorized, gin.H{"error": "unauthenticated"})
+		return
+	}
+
+	id := c.Param("id")
+	var localPath string
+	err := h.db.QueryRow(`SELECT local_path FROM repositories WHERE id = ? AND user_id = ?`, id, user.ID).Scan(&localPath)
+	if err == sql.ErrNoRows {
+		c.JSON(http.StatusNotFound, gin.H{"error": "repository not found"})
+		return
+	}
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+		return
+	}
+
+	result, err := services.PullRepo(localPath)
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+		return
+	}
+
+	h.db.Exec(`UPDATE repositories SET size_bytes = ?, last_used = NOW() WHERE id = ?`, result.SizeBytes, id)
+
+	c.JSON(http.StatusOK, gin.H{
+		"ok":       true,
+		"repo_id":  id,
+		"size_bytes": result.SizeBytes,
+	})
+}
+
 // CloneRepo handles POST /api/repos with SSE progress events.
 func (h *ReposHandler) CloneRepo(c *gin.Context) {
 	user := GetCurrentUser(c)