Storage: harden reconcile for concurrent writes and fix board transfer

[jeremy]

Reconcile now uses two-cursor approach: captures cursor before and after the storage scan, aborting if they differ (entries added during scan). Job retries 3x with 1-minute waits and limits concurrency to 1 per owner.

Board transfer now correctly moves storage for card description embeds and comment embeds, not just direct attachments. Uses batched queries to handle cards with thousands of comments efficiently.

Also fixes N+1 queries in attachment grouping via lookup maps.

/cc @jorgemanrubia