Add logic to compute scores

[nbars]

If there are exercises that are scored, we need logic that processes the data so that we can build, e.g., a scoreboard based on it.

We need these functions (besides maybe others):

• A function to get the best score for a specific exercise/task/user (or user group combination).
• Some function to get the highest and the lowest score for each task of each exercise. These values are needed to, e.g., normalize scores.
• Some way to provide per exercise (task?) formula to map the score values to a desired range.

[nbars]

Implemented in 7300ee0 on dev. Public scoreboard ships with switchable ranking strategies (f1_time_weighted, best_sum) and switchable views (default retro-terminal, minimal). Per-exercise scoring policies (linear/threshold/tiered) configured from the exercise edit form, group-aware team identity, live auto-refresh with persisted tab selections, and a unified site-wide navbar.