Persist document pages with revisions, hierarchy and item numbers #271

Work

• EF model and migration in the recall schema: page, revision, parent, sibling order.
• Allocate numbers from the shared project item sequence; item_get returns the new kind.
• Reparent and reorder atomically; reject cycles, including under concurrent moves.
• Deleting a page with children: decide between refusal, cascade and reparenting, then enforce it.
• Privacy deletion must cover pages like memories.

Acceptance

• Concurrent move A under B and B under A cannot both commit (SQL Server and SQLite).
• Numbers stay stable across moves and renames.
• Existing items and numbering are untouched by the migration.