feat: guest selections with per-person identity (#292)

Introduces a new "Per-guest selections" identity mode for event
feedback, letting each visitor register under their own name so their
likes/favorites/comments/ratings are tracked independently. Includes
admin insights (list, per-guest detail, aggregate view, export) and
advanced identity features (forget-me, email recovery, invite tokens,
merge).

New event-level setting
- event_feedback_settings.identity_mode = 'simple' | 'guest' (default
  'simple' → zero behavior change for existing events).
- Admin UI radio under Feedback Settings to toggle per event.

Root cause of the previous "all guests share state" bug
- generateGuestIdentifier() was sha256(ip + userAgent), so every visitor
  on the same WiFi + similar device collided into one identity.
- Now: when a verified guest JWT is present (x-guest-token header),
  req.guest.identifier takes precedence — per-person rate limits and
  per-person deduplication.

Phase 1 — identity layer
- Migration 078: new gallery_guests, guest_invites, guest_verification_
  codes tables; identity_mode column + check constraint; nullable
  guest_id FK on photo_feedback.
- New guest JWT type scoped to (eventId, guestId).
- New middleware guestAuth.resolveGuest (non-blocking) + requireGuest.
- POST /gallery/:slug/guest, GET /guest/me, DELETE /guest/me.
- Gallery feedback route enforces guest identity in guest mode and
  reads name/email from the verified token (never from the body).
- Frontend GuestIdentityContext + GuestNamePromptModal; axios
  interceptor injects x-guest-token on gallery API calls.
- Feedback-only blocking: gallery opens freely, prompt only on first
  interactive feedback action.
- Admin "Guests" tab (conditional on identity_mode='guest') with the
  AdminGuestsList component.

Phase 2 — admin insights
- GET /admin/events/:eventId/guests list + aggregated counts.
- GET /admin/events/:eventId/guests/:guestId detail with per-type
  groupings; AdminGuestDetail modal with thumbnail grid + tabs.
- GET /admin/events/:eventId/guests/aggregate sorted by distinct guest
  pick count; GuestSelectionsAggregate component.
- Per-guest export (txt/csv/json) and bulk export-all ZIP.

Phase 3 — polish
- 3.1 Self-service forget-me link in gallery footer.
- 3.2 Email-based identity recovery: POST /guest/recover sends a
  6-digit code via the existing emailProcessor, POST /guest/verify
  exchanges it for a token (rate-limited, enumeration-safe).
- 3.3 Admin invite tokens: pre-mint identities, share URLs with
  ?invite=, single-use redemption stripping the param from history.
- 3.4 Admin merge endpoint reassigns feedback + soft-deletes sources.

Shared helper
- useGalleryFeedbackAction hook wraps the identity-check logic for
  inline like buttons across Masonry/Grid/Justified/Mosaic/Carousel/
  Timeline/Premium layouts.

Backwards compatibility
- Existing events default to 'simple' after migration; behavior
  unchanged.
- Legacy photo_feedback rows keep guest_id NULL; admin shows them in
  the generic feedback moderation view as before.
- feedback_count denormalized stat now uses COALESCE(guest_id,
  guest_identifier) so per-guest counts are accurate without touching
  legacy rows.

Verified end-to-end against local Docker
- Migration clean on existing data.
- Simple mode unchanged (no prompt, legacy flow).
- Guest mode: Alice registers on click, tokens persist in
  sessionStorage, feedback rows carry guest_id.
- Carol via invite link auto-redeems, sees Alice's "1 likes" badge.
- Admin Guests tab shows both with correct counts; detail modal
  displays thumbnail grid with badges; aggregate view sorts by picker
  count (photo 227 = 2, others = 1); CSV/JSON export matches DB.
- Merge Carol into Alice: feedback reassigned, Carol soft-deleted,
  Alice count = 4.

backend/src/middleware/feedbackRateLimit.js

@@ -3,9 +3,20 @@ const { db } = require('../database/db');
 const logger = require('../utils/logger');
 
 /**
- * Generate a unique identifier for the guest
+ * Generate a unique identifier for the guest.
+ *
+ * In guest identity mode, `req.guest.identifier` is a server-issued UUID
+ * unique per person per event (set by the resolveGuest middleware). When
+ * present it takes precedence, so rate limits and deduplication become
+ * per-person instead of per-device.
+ *
+ * In simple (legacy) mode, the identifier falls back to a hash of IP + UA,
+ * matching prior behavior.
  */
 function generateGuestIdentifier(req) {
+  if (req.guest && req.guest.identifier) {
+    return req.guest.identifier;
+  }
   const ip = req.ip || req.connection.remoteAddress || 'unknown';
   const userAgent = req.headers['user-agent'] || 'unknown';
   return crypto