feat(feedback): export shape toggle — per-action vs per-guest pivot (#640 part E)

Ports 8digit/picpeak@ed7943b as a TOGGLE rather than a replacement. The
current per-action shape (one row per favourite/like/rating/comment) stays
the default for backward compat with any external scripts consuming the
export; the new pivot shape (one row per (photo, guest_identifier) with
boolean is_favorited/is_liked + star_rating + comment) is opt-in via a
?shape=pivot query param and a dropdown in the admin feedback page.

Pivot wins for "which guests engaged with which photos" analysis in
Sheets / Excel pivot tables. Long wins for engagement timeline analysis
and re-importing into another tool. Different products, both valid.

### Backend

- `feedbackService.exportEventFeedbackPivoted(eventId)`: new method.
  LEFT-of-Map approach, pure JS pivot so PG / SQLite behave identically.
  Key is `(filename, guest_identifier)` — anonymous guests with no
  identifier get a synthetic per-row key so two anonymous comments on the
  same photo don't collapse. Comments: most recent wins (history dropped
  in exchange for "current state" semantics). Hidden-by-moderator rows
  excluded — the pivot represents what we want to surface, not the raw
  event log.
- `adminFeedback.js` export route: accepts `?shape=pivot|long` (default
  `long`). CSV filename now carries the shape (e.g.
  `feedback-pivot-{id}.csv`) so repeated exports don't overwrite.
- `convertToCSV` helper in `adminFeedback.js` gains the three escaping
  improvements that 8digit's commit also shipped: booleans → `yes`/`no`,
  null/undefined → empty, escape strings containing newlines (\n/\r) as
  well as commas/quotes. Comments with line breaks were silently breaking
  CSV row counts before this. Improvements are pure wins regardless of
  shape; archives' own `convertToCSV` copy left untouched (separate
  surface, no behaviour drift risk).

### Frontend

- `feedback.service.ts` `exportEventFeedback()` gains optional `shape`
  parameter, default 'long'.
- `EventFeedbackPage.tsx`: new shape dropdown next to the CSV / JSON
  buttons (defaults to 'long'). Selected shape flows through to the API
  request AND the downloaded filename.

### i18n

3 new EN + DE entries (`feedback.exportShapeLabel`,
`feedback.exportShapeLong`, `feedback.exportShapePivot`).

### Notes

- Pivot shape is **per-guest current state**, not history. A guest who
  rated a photo, then changed their mind and removed the rating, would
  show the final state in the pivot but BOTH actions in the long form.
  Acceptable trade-off: pivot users care about the snapshot, long users
  want the trail.
- `latest_at` column in pivot gives a "most recent activity" timestamp
  per row, useful for sorting/filtering recent engagement.

### Test plan

- [x] Backend syntax + TS check + lint clean (no new warnings; existing
      `catch (error)` warning was pre-existing)
- [ ] Manual: feedback page → select Per-guest (pivot) → Export CSV →
      verify one row per (filename, guest) with is_favorited='yes'/'no',
      latest_at column populated
- [ ] Manual: long shape default still produces the same per-action
      output as before (no regression for existing consumers)
- [ ] Manual: comment containing a newline → pivot CSV escapes correctly,
      row count matches data length + 1 header
- [ ] Manual: archive a published event with feedback → archive's
      `feedback_data.csv` still uses the long shape (archive surface
      unchanged on purpose)

frontend/src/services/feedback.service.ts

@@ -136,9 +136,18 @@ class FeedbackService {
     return response.data;
   }
 
-  async exportEventFeedback(eventId: string, format: 'json' | 'csv' = 'json') {
+  // `shape` defaults to 'long' (one row per feedback action) for backward
+  // compatibility with anyone scripting against this endpoint. 'pivot' (per
+  // #640 #6) returns one row per (photo, guest_identifier) with boolean
+  // is_favorited / is_liked plus star_rating + comment. Hidden-by-moderator
+  // rows are excluded from the pivot.
+  async exportEventFeedback(
+    eventId: string,
+    format: 'json' | 'csv' = 'json',
+    shape: 'long' | 'pivot' = 'long',
+  ) {
     const response = await api.get(`/admin/feedback/events/${eventId}/feedback/export`, {
-      params: { format },
+      params: { format, shape },
       responseType: format === 'csv' ? 'blob' : 'json'
     });
     return response.data;