Store feedback grids as one validated object

[jaykbpark]

Summary

Store each feedback grid as one question with grid.rows and grid.columns, replacing duplicated per-row grouping metadata. Answers remain flat and keyed by stable row IDs.

• Normalize grid definitions at the persistence boundary with short validation guards and specific errors.
• Document the stored shape beside its type. Validate row IDs, labels, limits and collisions; preserve column labels containing commas.
• Remove the standalone backend test file, which has no maintained CI test command.
• Add AGENTS.md guidance on simple data shapes, boundary validation, readable guards, documentation and existing test conventions.

Contract

{
  questionId: "event-quality",
  type: "MULTIPLE_CHOICE_GRID",
  label: "Rate the event",
  required: true,
  grid: {
    rows: [{ id: "organization", label: "Organization" }],
    columns: ["Poor", "Excellent", "Yes, definitely"]
  }
}
// Answers remain flat: responses.organization = "Excellent"

Verification

• Focused strict TypeScript check passes for the event types and feedback helpers.
• Local checks pass for both form types, invalid definitions, flat required/optional answers, stable IDs, comma-containing choices and unchanged ordinary questions.
• The companion frontend browser checks exercise the real normalization and answer validators with synthetic data. One-off verification files stay outside this PR.
• Formatting and diff checks pass.

Rollout

Deploy this API PR before frontend PR #504, which uses the new question type. No production form or response was changed.

[jimmysamportfolio]

LGTM - pay attention to merge order. Also, fix the failing pr rule