Lock available tasks for a specific occupation and prolific_id.

Parameters:

• occupation_id: The occupation ID to filter tasks
• prolific_id: The prolific ID to assign tasks to
• limit: Maximum number of task groups to return (default: 6). Each group contains 1-3 task instances. For example, limit=6 returns 6 groups with a total of 6-18 tasks.
• preview: Preview mode flag (default: True)
  • True: Returns available tasks without locking them. No database changes are made.
  • False: Actually locks the tasks in the database and assigns them to the prolific_id.

Task Grouping Logic:

• Tasks are formatted as xxx-1, xxx-2, xxx-3, xxx-4, xxx-5, xxx-6
• Tasks are grouped by prefix (xxx)
• Only available tasks are fetched. Completed groups (groups with >= 2 unavailable task instances) are excluded at the database level and are never returned.
• For each prefix returned, up to 3 newest task instances are selected (created_at desc)
• Groups are returned in newest-first order (max created_at in the group)
• Each group will have at least 1 task instance and at most 3 task instances
• If a prefix has more than 3 available tasks, the 3 newest are selected
• If a prefix has 1-3 available tasks, all are included
• Up to 'limit' groups are returned (e.g., limit=6 returns 6 groups)
• Total tasks returned will be between 6 (6 groups × 1 task min) and 18 (6 groups × 3 tasks max)

Example:

• Prefix 1: has 3 available tasks (e.g. 1-1, 1-2, 1-4) → group is included with those 3 tasks
• Prefix 2: is completed (>= 2 unavailable) → group is not fetched at all; only groups with < 2 unavailable are returned

Returns: TaskGroupResponse containing grouped tasks with metadata (total_groups, total_tasks).