Fix Nextcloud sync conflict by using content-hash ETags

The Nextcloud Android client compares ETags before and after upload to
verify its write landed. OxiCloud was returning the stable file UUID as
the ETag, which never changed on content updates, causing false
SYNC_CONFLICT errors on every upload.

Five fixes applied:
1. Thread blob_hash (SHA-256) through File entity, FileDto, all read/write
   queries, and all WebDAV/PROPFIND responses as the ETag — changes on
   every content update, no DB migration needed.
2. Honor X-OC-Mtime header: parse the client-supplied mtime and use it
   for updated_at via COALESCE(to_timestamp($n), NOW()).
3. Disable phantom checksum capability (preferredUploadType/supportedTypes)
   that the server never actually implemented, stopping retry loops.
4. Add nc:creation_time and nc:upload_time to PROPFIND responses.
5. Return oc-etag header in chunked upload MOVE (assemble) responses.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/interfaces/api/handlers/webdav_handler.rs

@@ -928,6 +928,7 @@ async fn handle_put(
             total_bytes as u64,
             &content_type,
             Some(hash),
+            None,
         )
         .await;