Fix Ten Things list value delete/clear causing 500

[FlandersBurger]

Problem

Deleting or blanking an answer in the Ten Things list editor was broken: the trash icon didn't persist, and clearing an answer's text returned a 500 from the API.

Root causes

• Trash icon didn't save: removeValue in ListEditor.tsx only updated local React state via onChange, never triggering a save — unlike every other edit in the component (name, categories, language, difficulty, value/blurb text). Deletions looked like they worked but were silently lost.
• Clearing a value 500'd: PUT /api/tenthings/lists/:id called list.validate()/list.save() with no error handling. Mongoose's required: true check rejects empty strings for the value's value field, so blanking an answer and blurring threw an unhandled ValidationError that fell through to the app's catch-all handler as a bare 500.

Fix

• removeValue now also fires a save, matching the pattern used elsewhere in the editor.
• PUT /:id now rejects blank answers with a proper 400 { error: "Answers cannot be blank." } before saving, and wraps validate()/save() in try/catch returning 400 with the real validation message instead of crashing to 500.
• Applied the same blank-answer guard to POST / (list creation), which shares the same shape of bug.

Testing

• npx jest --selectProjects server --testPathPatterns="tenthings/lists" — 15/15 passing.
• Confirmed Mongoose's required-string behavior with a standalone repro (rejects "").
• Not manually verified end-to-end against a live dev server/DB in this environment — recommend a quick click-through (delete a value, clear-and-blur a value) before merge.

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