feat(gradebook): add Level contribution to weighted gradebook

[LWS49]

Summary

Adds an optional Level contribution to the weighted gradebook, sitting alongside the existing assessment-tab weights. An instructor enters an arithmetic formula (e.g. level * 2 or min(level, 5)) that maps each student's course Level to grade-points, which then folds into the student's weighted Total. The formula, weight budget, effective max level, and column visibility are configured in the Configure Contributions dialog and persisted per course. A new Level column surfaces each student's contribution in the weighted table.

Design decisions

• Formula handling parses into a small arithmetic AST and evaluates with plain arithmetic rather than eval - a hostile or malformed string can only ever produce a parse error, never run as code. The grammar is restricted to numbers, the level/maxLevel variables, the four operators, and floor/ceil/round/min/max.
• Level config is a singleton per course (course_id unique) upserted on save, not a row-per-edit, since there is exactly one Level contribution per course.
• The Level term reuses the existing breakdown structure via synthetic negative tab/assessment ids (-1), keeping it disjoint from real positive assessment ids so it flows through the same compute and rendering paths without a parallel code path.
• weight is treated as an advisory budget: contributions outside [0, weight] drive a dialog warning rather than being clamped, leaving the instructor in control of the formula.

Regression prevention

• Covers: formula tokenizer/parser including precedence, unary minus, functions, and rejection of invalid input (levelFormula.test.ts); Level folding into student totals and breakdown, null/disabled handling, and out-of-range detection (computeWeighted.test.ts); store hydration and update of levelContribution/courseMaxLevel (store.test.ts); dialog enable/configure/warning behaviour (ConfigureWeightsPrompt.test.tsx); Level column rendering (GradebookWeightedTable.test.tsx); LevelConfig validations and upsert_for singleton behaviour (level_config_spec.rb); controller persistence and serialization (gradebook_controller_spec.rb).
• Manually tested: enabling and configuring a formula with weight/max level and confirming it persists across reload; Level column and Total reflecting the contribution and disabling removing it; invalid formula handled gracefully without crashing the Total; out-of-range warning firing; show toggle controlling column visibility independent of enabled; weight-only saves (no levelContribution param) leaving the singleton config untouched.
• Backward compatible: when the weighted view is disabled or no Level config exists, the serializer returns a disabled/zeroed contribution and the gradebook behaves exactly as before.

Adding screenshots: the Configure Contributions dialog showing the Level section (formula input + weight + out-of-range warning), and the weighted table with the Level column populated.