S3: map canned ACL to file permissions and add configurable default file mode (#8886)

* S3: map canned ACL to file permissions and add configurable default file mode

S3 uploads were hardcoded to 0660 regardless of ACL headers. Now the
X-Amz-Acl header maps to Unix file permissions per-object:
- public-read, authenticated-read, bucket-owner-read → 0644
- public-read-write → 0666
- private, bucket-owner-full-control → 0660

Also adds -defaultFileMode / -s3.defaultFileMode flag to set a
server-wide default when no ACL header is present.

Closes #8874

* Address review feedback for S3 file mode feature

- Extract hardcoded 0660 to defaultFileMode constant
- Change parseDefaultFileMode to return error instead of calling Fatalf
- Add -s3.defaultFileMode flag to filer.go and mini.go (was missing)
- Add doc comment to S3Options about updating all four flag sites
- Add TestResolveFileMode with 10 test cases covering ACL mapping,
  server default, and priority ordering

weed/command/filer.go

@@ -146,6 +146,7 @@ func init() {
 	filerS3Options.iamReadOnly = cmdFiler.Flag.Bool("s3.iam.readOnly", true, "disable IAM write operations on this server")
 	filerS3Options.portIceberg = cmdFiler.Flag.Int("s3.port.iceberg", 8181, "Iceberg REST Catalog server listen port (0 to disable)")
 	filerS3Options.externalUrl = cmdFiler.Flag.String("s3.externalUrl", "", "the external URL clients use to connect (e.g. https://api.example.com:9000). Used for S3 signature verification behind a reverse proxy. Falls back to S3_EXTERNAL_URL env var.")
+	filerS3Options.defaultFileMode = cmdFiler.Flag.String("s3.defaultFileMode", "", "default file mode for S3 uploaded objects, e.g. 0660, 0644, 0666")
 
 	// start webdav on filer
 	filerStartWebDav = cmdFiler.Flag.Bool("webdav", false, "whether to start webdav gateway")