Add atomic task deduplication with partial unique index

- Add partial unique index for active resource-scoped tasks
- Implement createTaskRunRecordAtomic for race-free task creation
- Update findOrEnqueueTask to use atomic insert first
- Add tests for concurrent task creation deduplication

drizzle/0013_task_active_resource_unique.sql

@@ -0,0 +1,3 @@
+CREATE UNIQUE INDEX IF NOT EXISTS `task_active_resource_uidx` 
+ON `task_run` (`user_id`, `task_type`, `resource_key`) 
+WHERE `resource_key` IS NOT NULL AND `status` IN ('queued', 'running');