filer: keep empty folders that are s3tables catalog entries (#11102)

* s3tables: build the catalog attribute keys from one shared prefix

Every attribute the catalog stores on a bucket, namespace, table or view
entry is spelled out with the same literal prefix. Name it once in
s3_constants so code outside the package can recognize a catalog entry
without repeating the string.

Claude-Session: https://claude.ai/code/session_01GfZsc4cyNB2yr6KYLRv9q1

* filer: keep empty folders that are s3tables catalog entries

A namespace, table or view is a directory whose extended attributes are
the catalog record. Its files can live elsewhere - a rename moves only
the catalog pointer and leaves the data at the old path, and a view has
no files at all - so an empty one is still a live entry.

Drop a table, then rename another table onto that name: the drop queues
the old table's folders, the rename recreates the name path, and two
minutes later the cleaner deletes it and cascades into the namespace,
losing a table the catalog still lists.

Claude-Session: https://claude.ai/code/session_01GfZsc4cyNB2yr6KYLRv9q1

* filer: drop a queued cleanup when the folder is created again

A cleanup is queued against the folder that was found empty. If that
folder is deleted and a new one takes its name, the queue entry outlives
the folder it was about and the next pass deletes the replacement. A
drop followed by a rename onto the dropped name does exactly this: the
name path comes back as a live catalog entry two minutes before the
queue is read.

Claude-Session: https://claude.ai/code/session_01GfZsc4cyNB2yr6KYLRv9q1

weed/s3api/s3_constants/extend_key.go

@@ -49,6 +49,10 @@ const (
 	// Bucket Policy
 	ExtBucketPolicyKey = "Seaweed-X-Amz-Bucket-Policy"
 
+	// Every attribute the s3tables catalog stores on a table bucket, namespace,
+	// table or view directory entry starts with this.
+	ExtS3TablesPrefix = "s3tables."
+
 	// Object Retention and Legal Hold
 	ExtObjectLockModeKey     = "Seaweed-X-Amz-Object-Lock-Mode"
 	ExtRetentionUntilDateKey = "Seaweed-X-Amz-Retention-Until-Date"