feat(gradebook): external assessment management panel and drag-and-drop reorder

[LWS49]

Summary

Gives external assessments a proper management home. A single page-level "Manage external assessments" panel (above the tabs) lists every external (Name / Max / Weight / Actions) and hosts Add, Edit, and Delete; it replaces what would otherwise be scattered toolbar buttons and an in-column kebab menu. Externals can be reordered by drag-and-drop, persisted via a new position column and served in canonical order. Adding or editing an external with a non-zero weight while the equal-split default is showing freezes that split first, so setting one real weight does not collapse every other tab to zero.

Design decisions

• Management lives in one panel gated on canManageWeights but not on weightedViewEnabled - recording a paper-exam grade is valid even with weighting off, so externals must stay manageable; the Weight column is read-only and shown only when the weighted view is on (weight editing stays in Configure Weights, to avoid two edit paths).
• All rename/delete live in the panel rather than an in-table column kebab - this removes the add-without-delete and toolbar-add-vs-buried-delete asymmetries; the column header keeps only an "External" chip for identification.
• Adding/editing an external with a non-zero weight while the table shows the equal-split default persists that visible split first (via the existing updateWeights endpoint and the materializedDefaultWeights helper) - otherwise setting one real weight would disengage the fallback and collapse every other tab to its stored 0. No-op when weights are already configured or the new weight is 0.

Regression prevention

Covers: the manage panel (list, add/edit/delete, empty state), the delete prompt (confirm / failure-toast / cancel paths), the edit prompt (rename, max, floor/cap toggles), drag-reorder dispatch and failure rollback, the default-weight materialization on add/edit, and the related gradebook operation thunks and store reducers. Adds the position migration (20260624, reversible); existing externals get sequential positions on migrate, so the change is backward compatible. External-assessments controller and model specs run on CI.

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♻️ Recreated after #8463 was auto-closed as "merged" when the base branch was briefly force-pushed to a commit that contained this branch. Same head, base, and content; the base has been restored.