Add views for blocked and needs-intervention tasks

Problem: Need quick access to tasks that are blocked or have failed 3+ times.

Fix:

• Add GET /blocked route - shows tasks with unmet blocking dependencies
• Add GET /intervention route - shows tasks with retryAttempt >= 3 in retry_context
• Add links to these from dashboard

Backend changes needed in Omni/Task/Core.hs:

• getBlockedTasks :: IO [Task]
• getInterventionTasks :: IO [Task]