fix(vfss3): stop rejecting all uploads when no quota is set

CheckAvailableDiskSpace returns maxsize = fs.MaxFileSize() when no
disk quota is configured. s3VFS.MaxFileSize() returned 0 (intended as
"no per-file limit"), but that meant CreateFile's redundant
`if newsize > maxsize` post-check fired for every non-empty upload —
making any file fail with ErrFileTooBig → HTTP 413 → "storage full"
in the UI, even on an empty volume.

Two changes:
 - Return -1 from MaxFileSize() to match aferoVFS's convention for
   "no limit". This makes the streaming-time check in
   s3FileCreation.Write (`f.maxsize >= 0 && f.w > f.maxsize`) skip
   correctly when no quota is in effect.
 - Drop the redundant post-CheckAvailableDiskSpace `newsize > maxsize`
   check from CreateFile and CopyFileFromOtherFS — the function
   already returns the right error when relevant.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Crash--

model/vfs/vfss3/impl.go

@@ -142,7 +142,7 @@ func New(db vfs.Prefixer, index vfs.Indexer, disk vfs.DiskThresholder, mu lock.E
 }
 
 func (sfs *s3VFS) MaxFileSize() int64 {
-	return 0 // no per-file limit — S3 multipart handles large files transparently
+	return -1 // no per-file limit — S3 multipart handles large files transparently
 }
 
 func (sfs *s3VFS) DBCluster() int {
@@ -245,9 +245,10 @@ func (sfs *s3VFS) CreateFile(newdoc, olddoc *vfs.FileDoc, opts ...vfs.CreateOpti
 	if err != nil {
 		return nil, err
 	}
-	if newsize > maxsize {
-		return nil, vfs.ErrFileTooBig
-	}
+	// CheckAvailableDiskSpace already returns ErrFileTooBig / ErrMaxFileSize
+	// when the upload would exceed the per-file or quota budget. The
+	// streaming-time check below (s3FileCreation.Write) re-checks against
+	// maxsize once the actual byte count is known.
 
 	if olddoc != nil {
 		newdoc.SetID(olddoc.ID())
@@ -703,13 +704,10 @@ func (sfs *s3VFS) CopyFileFromOtherFS(
 	}
 	defer sfs.mu.Unlock()
 
-	newsize, maxsize, capsize, err := vfs.CheckAvailableDiskSpace(sfs, newdoc)
+	newsize, _, capsize, err := vfs.CheckAvailableDiskSpace(sfs, newdoc)
 	if err != nil {
 		return err
 	}
-	if newsize > maxsize {
-		return vfs.ErrFileTooBig
-	}
 
 	newpath, err := sfs.Indexer.FilePath(newdoc)
 	if err != nil {