Add Parked tab and enforce strict board routing

[trsdn]

Problem

The board routing is still too loose.

The Backlog tab should represent the ready backlog, but items from other states still leak into it. At the same time, the board is missing a dedicated Parked tab even though we now use status:parked.

Desired behavior

Each board tab should show only its own category, and the Backlog tab should contain only ready backlog items.

Acceptance criteria

• Add a Parked tab alongside Backlog / Decisions / Ideas.
• The Backlog tab shows only ready backlog items.
• The Backlog tab excludes blocked items, decision-required items, idea items, and parked items.
• The Blocked tab shows only blocked items.
• The Decisions tab shows only decision-required items.
• The Ideas tab shows only idea items.
• The Parked tab shows only parked items.
• Filtering is enforced server-side and not only in the client UI.
• Regression coverage is added for the routing/filtering rules.