Fix watcher deletion sync bugs and improve file download resilience

[zephyranthes03]

• Fix watcher deletion using wrong IDs (client ID → server ID mapping)

  • get_all_groups_with_watchers now returns server watcher IDs
  • batch_soft_delete_watchers uses server_watcher_id column
  • get_all_soft_deleted_items uses correct JOIN column (group_id)

• Fix SyncConfiguration response missing data

  • Populate deleted_items (field 12) from explicit/diff-based deletions
  • Track files_soft_deleted count for individual watcher deletes
  • Include soft-delete timestamps in server_latest_ts calculation via new get_latest_config_timestamp storage method

• Improve file download error handling

  • Unary handler: pre-check data existence before 'too large' redirect
  • Streaming handler: fallback to FileService when storage stream returns None (handles S3 migration gaps)
  • Add diagnostic logging for storage type and file size on failures