fix: add enabled:true to all graders, restore /tasks dict format, add per_task_rewards alias

fix: return flat list from GET /tasks so platform grader check passes

Iteration_4: expand task bank 20→30, add variants for all tasks, fix baseline policy for chain-gated CRIT tasks

fix: resolve all 3 pre-submission critical risks

chore: pre-iteration-1 snapshot — 75/75 e2e passing, baseline bug fixed