Add Backlog tab filters for blocked and decision-required items

[trsdn]

Problem

The Backlog tab needs explicit filter controls so I can decide whether decision-required and blocked items should also appear there while triaging.

Right now those items are routed into dedicated tabs, but there is no simple way to include or exclude them from the Backlog view when I want a broader backlog picture.

Desired behavior

Add Backlog filter controls for:

• items labeled needs:stakeholder-decision
• items labeled status:blocked

Acceptance criteria

• The Backlog tab exposes visible filter controls for decision-required and blocked items.
• I can independently toggle whether needs:stakeholder-decision items are shown.
• I can independently toggle whether status:blocked items are shown.
• Default behavior stays behavior-safe and does not unexpectedly flood the backlog with blocked/decision items.
• Changing a filter updates the visible list immediately without a full page reload.
• Regression coverage is added for the selection/filtering logic.