Allow members to see tasks from repos they have access to

[jrchatruc]

Summary

• Members could only see tasks they personally created, even if they had access to the repo
• Now check_task_ownership also checks repo permissions — if a user has access to the task's repo (via org membership or explicit grant), they can view the task
• Task list also updated to show tasks from accessible repos, not just tasks the user created
• Affects all task endpoints: detail, conversations, logs, diff, actions, PR creation, reopen

Test plan

• As admin, grant a member access to a repo in Settings
• As that member, verify they can see tasks from that repo created by other users
• Verify they can view conversations, logs, diff, and other task data
• Verify admins still see everything
• Verify viewers only see tasks from repos explicitly granted to them