Implement VFS quota support

- Add VfsQuota and VfsQuotaUsage structs
- Add quota methods to VfsBackend trait:
  - set_quota: set space/file limits
  - get_quota: retrieve quota settings
  - get_quota_usage: current usage stats
  - check_quota: pre-write check
- Implement LocalFs quota support:
  - Uses .quota metadata file
  - JSON storage for quota limits
  - Recursive size/file counting
  - Hidden files excluded (.quota, .snapshots)

Enables SMB per-share/user quota enforcement.

All 229 tests pass.

markbase-core/src/vfs/mod.rs

@@ -195,6 +195,28 @@ pub trait VfsBackend: Send + Sync {
     fn snapshot_info(&self, _path: &Path, _name: &str) -> Result<VfsSnapshotInfo, VfsError> {
         Err(VfsError::Unsupported("snapshot_info".to_string()))
     }
+
+    // ===== Quota support =====
+
+    /// 设置配额限制（字节）
+    fn set_quota(&self, _path: &Path, _quota: &VfsQuota) -> Result<(), VfsError> {
+        Err(VfsError::Unsupported("set_quota".to_string()))
+    }
+
+    /// 获取配额信息
+    fn get_quota(&self, _path: &Path) -> Result<VfsQuota, VfsError> {
+        Err(VfsError::Unsupported("get_quota".to_string()))
+    }
+
+    /// 获取配额使用情况
+    fn get_quota_usage(&self, _path: &Path) -> Result<VfsQuotaUsage, VfsError> {
+        Err(VfsError::Unsupported("get_quota_usage".to_string()))
+    }
+
+    /// 检查配额（写入前检查）
+    fn check_quota(&self, _path: &Path, _size: u64) -> Result<bool, VfsError> {
+        Ok(true) // Default: no quota, always allow
+    }
 }
 
 /// 快照信息
@@ -209,3 +231,31 @@ pub struct VfsSnapshotInfo {
     /// 是否只读
     pub read_only: bool,
 }
+
+/// 配额设置
+#[derive(Debug, Clone)]
+pub struct VfsQuota {
+    /// 空间限制（字节），0表示无限制
+    pub space_limit: u64,
+    /// 文件数量限制，0表示无限制
+    pub file_limit: u64,
+    /// 用户ID（可选）
+    pub user_id: Option<String>,
+    /// 软限制（字节），超过时警告
+    pub soft_limit: u64,
+    /// 宽限期（秒）
+    pub grace_period: u64,
+}
+
+/// 配额使用情况
+#[derive(Debug, Clone)]
+pub struct VfsQuotaUsage {
+    /// 已使用空间（字节）
+    pub space_used: u64,
+    /// 文件数量
+    pub files_used: u64,
+    /// 是否超过软限制
+    pub over_soft_limit: bool,
+    /// 是否超过硬限制
+    pub over_hard_limit: bool,
+}