feat(gradebook): external assessments with bounded grades and CSV import

[LWS49]

Summary

Adds external assessments to the gradebook, the only gradebook columns a professor can create, edit, and delete, on top of the weighted gradebook view (#8436). Each external assessment carries optional floor/cap bound flags, ships with a CSV grade-import core, and stores grades at decimal(5,2) so two-decimal imports (Canvas, SoftMark) round-trip without silent rounding. Also adds the materializedDefaultWeights helper, so a later mutating action that sets a real weight while the equal-split default is showing can freeze that split first (wired up in the next PR).

Design decisions

• External grade/max columns use decimal(5,2); native grade columns stay decimal(4,1) - externals are fed by Canvas/SoftMark which emit two decimals, while native grading is Coursemology-native where one decimal is fine. Widening every grade column would be a larger, separate change.
• Floor and cap flags are applied only at weighted-compute time (in effectiveGrade), never mutating the stored grade - the raw "All assessments" view always shows the true entered value, and toggling a flag re-interprets rather than rewrites data.

Regression prevention

Covers: external assessment column-id building, the external assessment and external grade models, store/operations reducers, the import wizard, gradebook table rendering, and the new materializedDefaultWeights helper (frontend suites green - 262 tests). Migrations (20260615 create external tables, 20260616 link externals to contributions, 20260622 bounds, 20260623 decimal(5,2) precision) are reversible. Ruby model/controller specs run on push/CI.