P3: Quota enforcement - check before write in flush()

- Check VfsBackend quota before writing buffered data
- Return FsError::InsufficientStorage (507) if limit exceeded
- Log warning with current/adding/limit values

Tests: 289 passed, 0 failed

markbase-core/src/webdav.rs

@@ -510,6 +510,30 @@ impl DavFile for VfsDavFile {
             return Box::pin(std::future::ready(Ok(())));
         }
 
+        // Quota check before write
+        if !self.data.is_empty() {
+            if let Some(vfs) = &self.vfs {
+                if let Some(path) = &self.path {
+                    let usage = vfs.get_quota_usage(path);
+                    let quota = vfs.get_quota(path);
+                    if let (Ok(usage), Ok(quota)) = (usage, quota) {
+                        if quota.space_limit > 0 {
+                            let new_size = usage.space_used + self.data.len() as u64;
+                            if new_size > quota.space_limit {
+                                log::warn!(
+                                    "Quota exceeded: current={}, adding={}, limit={}",
+                                    usage.space_used,
+                                    self.data.len(),
+                                    quota.space_limit
+                                );
+                                return Box::pin(std::future::ready(Err(FsError::InsufficientStorage)));
+                            }
+                        }
+                    }
+                }
+            }
+        }
+
         // Phase 1: Flush to storage
         if let Some(vfs_file_mutex) = &self.vfs_file {
             match vfs_file_mutex.lock() {