feat(gradebook): weighted view with weights and exclusions

[LWS49]

Summary

Adds an opt-in weighted gradebook view on top of the base gradebook. Course staff enable it through a new Gradebook settings toggle, then configure per-tab weights in either equal mode (every included assessment counts equally toward the tab subtotal) or custom mode (explicit per-assessment weights that must sum to the tab total). Totals are additive: each tab subtotal is scaled by its weight and the gradebook Total is the literal sum of those subtotals. Staff can exclude individual assessments from totals . A per-student breakdown popover, a points/percentage display toggle, and a projected-total hint complete the view.

Design decisions

• Weighted view is opt-in per course via a settings toggle - courses that don't want weighting see the existing gradebook unchanged.
• Custom-mode weights are validated server-side to sum (at 2 decimal places) to the tab total, inside a transaction that rolls back on mismatch - prevents partially-saved weight sets that don't add up.
• Exclusion flags are rewritten across the whole tab on every save (excluded ids to true, the rest to false) - avoids stale exclusions when modes change or assessments move.

Regression prevention

• Backend specs: the tab weight-update pipeline (equal/custom modes, custom sum gate, exclusions, drop-lowest, unknown tab/assessment, transactional rollback), gradebook controller (index JSON plus update_weights authorization and error paths), gradebook ability, and the gradebook settings component.
• Frontend tests: computeWeighted (equal/custom math, exclusions, drop-lowest), GradebookWeightedTable, the configure-weights prompt, weighted column tree, projected-total and weighted-view hints, the store, and the gradebook settings form.
• Manual testing (confirmed by author): enabling and disabling the view, equal and custom weight configuration including the sum gate, per-assessment exclusion, points/percentage toggle, breakdown row expansion, and permission gating for read vs manage.
• Backward compatible: new columns default to non-weighted behaviour (tab weight 0, equal mode, not excluded) and the weighted view stays hidden unless explicitly enabled, so existing gradebook behaviour is unchanged.