Add post-event feedback forms

A Feedback step in the event wizard builds a survey that is sent at a
chosen time to everyone who checked in (or everyone registered), with
personal links and one reminder after 3 days. Answers can be anonymous.
Adds rating (1-5 stars) and multiple-choice field types, also usable on
attendee forms, and a Feedback summary report with response rate,
average ratings, choice breakdowns, comments and event comparison.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/API_DOCUMENTATION.md

@@ -25,6 +25,7 @@ Tokens are issued by `POST /api/users/login`.
 - [Ticket transfer](#ticket-transfer)
 - [Rooms & groups](#rooms--groups)
 - [Sponsor a spot](#sponsor-a-spot)
+- [Post-event feedback](#post-event-feedback)
 - [Payments](#payments)
 - [Tickets](#tickets)
 - [Broadcasts (Email)](#broadcasts-email)
@@ -282,6 +283,22 @@ For unauthenticated (guest) registration on events with `requiresAuth: false`:
 
 ---
 
+## Post-event feedback
+
+| Method | Path | Auth | Description |
+|--------|------|------|-------------|
+| GET | `/api/feedback/event/:eventId` | supervisor+ | The event's feedback form (with `invited` / `responses` counts), or null |
+| PUT | `/api/feedback/event/:eventId` | supervisor+ | `{ sendAt, audience: 'checked_in'\|'all', anonymous, intro, fields[] }` — fields can't change once answered (409) |
+| DELETE | `/api/feedback/event/:eventId` | supervisor+ | Remove the form |
+| POST | `/api/feedback/event/:eventId/send-now` | supervisor+ | Send now (also invites anyone not yet invited) |
+| GET / POST | `/api/feedback/public/:token` | link holder | Load the form / submit `{ answers: { fieldId: value } }` once |
+| GET | `/api/feedback/summary?eventIds=a,b` | supervisor+ | Per event: invited, responses, response rate, and per question: rating average + distribution, choice counts, text answers (names only when not anonymous) |
+
+Notes:
+- New field types `rating` (answers `"1"`–`"5"`) and `multiple_choice` (`options`: array of strings). They can be used on attendee forms too.
+- Only HMACs of link tokens are stored, and a reminder issues a new link.
+- A job every 15 minutes (`FEEDBACK_JOB_ENABLED`, `FEEDBACK_JOB_INTERVAL_MS`) sends due forms, and sends one reminder 3 days later to people who haven't answered.
+
 ## Sponsor a spot
 
 | Method | Path | Auth | Description |