Cognito Forms

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Build me the form builder with real logic that replaces Cognito Forms.

The distinguishing features are calculations, repeating sections and save-and-resume. Each has a specific trap and they are what this prompt is about.

STACK
- Node 20+ with Fastify, server-rendered HTML with a little vanilla JS
- SQLite through better-sqlite3, WAL mode
- Caddy in front

THE DATA MODEL
- forms: id, slug, title, schema_json, version, is_open
- entries: id, form_id, form_version, values_json, status, created_at, submitted_at, resume_token
- **Store the form version on every entry.** Editing a form after entries exist otherwise makes the old ones unreadable, and this is the bug that eats a whole dataset
- payments: id, entry_id, provider_ref, amount_cents, currency, status, at

CALCULATIONS, DONE SAFELY
- A small expression language over field names: arithmetic, comparison, a few functions. **Never eval user input** — write a tiny parser or use a sandboxed expression library
- **Compute on the server and treat the browser's copy as a convenience only.** A price calculated in the browser is a price the visitor can change, and if it feeds a payment you have built a shop with editable prices
- Money in integer minor units, rounded once at the end, with the rounding rule stated
- Show the working: a total that appears with no explanation gets disputed

REPEATING SECTIONS
- An array of objects in the values, with a minimum and maximum count
- Every field inside gets a stable index in its name, and validation errors point at the right row
- Aggregate across rows in calculations — sum, count — which is why people want repeating sections at all
- **Adding a row must not lose what is typed in the others.** Server-render the state or keep it in the DOM; do not re-fetch

SAVE AND RESUME
- A token URL emailed to the person, entry status 'draft', and an expiry
- **The token is a key to their data: make it long and random, expire it, and never put it in a URL that is logged.** A guessable resume link exposes everybody's half-finished forms
- Autosave on a debounce, with a visible saved indicator

CONDITIONAL LOGIC
- Show and hide by rule, evaluated on both sides, and **hidden fields are not validated and not stored.** A required field hidden by logic that still blocks submission is the most common broken form on the internet

PAYMENTS
- Create the payment intent on the server from the server-computed amount, never from a number the browser sent
- The entry is only complete when the provider confirms, by webhook, verified

WHAT MATTERS MOST
Server-side calculation and the form version on every entry. One stops the totals being edited; the other stops your data becoming unreadable.