Add global category management with parent rollups

[theMattCode]

User story

As a user, I want to manage global finance categories so cashflow and budgets can be reported consistently across accounts.

Depends on

• Issue Build finance foundation schema and backend module structure #12

Scope

• Add backend APIs for listing, creating, updating, and archiving categories.
• Support optional parent category relationship.
• Prevent parent category cycles.
• Keep categories global across finance, not account-scoped.
• Add Cockpit category management UI.
• Ensure budget allocations target specific child categories, not parent rollups.

Non-scope

• Tags or labels.
• Budget allocation UI.

Acceptance criteria

• User can create flat categories.
• User can optionally assign a parent category.
• Parent categories are used for reporting rollups only.
• Used categories can be archived, not hard deleted.
• Archived categories are hidden from active entry forms but retained for history.