feat: native S3 storage backend (#328) + presigned download follow-up

Lets PicPeak write photos, thumbnails, hero images, watermarks, and
archive zips to any S3-compatible bucket (AWS S3, MinIO, Cloudflare R2,
Backblaze B2, Wasabi, DigitalOcean Spaces) instead of the local
filesystem. Selected via STORAGE_BACKEND=local|s3.

Architecture
- backend/src/services/storage/StorageBackend.js — abstract interface
  (put/get/exists/stat/delete/list/copy/rename/signedUrl/putFromFile/
  getToFile) — typedef-only, documents the contract.
- LocalFsStorage.js — wraps fs with atomic-write-via-tmp-rename, path
  traversal protection, list-as-walker.
- S3StorageBackend.js — thin wrapper around the existing
  S3StorageAdapter (used by backupService) mapping it onto the canonical
  interface; supports optional STORAGE_S3_PREFIX namespace.
- index.js — factory selected by STORAGE_BACKEND with startup ping
  (HEADs sentinel key on S3, fs.stat on local) so misconfig fails fast
  before the first request.

Consumer refactors (~12 services + routes), each parametrized over the
abstraction:
- imageProcessor / videoProcessor — pipe Sharp/ffmpeg output through
  storage.put; expose withLocalCopy() helper for S3-mode regeneration
  paths that need a local file for sharp/ffmpeg.
- archiveService / downloadZipService — finalize zip in tmp dir, then
  storage.putFromFile. Atomic-rename pattern preserved on local; S3
  emulates via copy + delete (worker prunes orphaned .tmp.* on startup).
- photoProcessor / photoReplacementService / adminPhotos upload+delete /
  routes/v1/events.js POST /events/:id/photos / routes/events.js — every
  upload path now goes storage.putFromFile(temp) → unlink temp.
- gallery.js bulk-download (cached + on-the-fly + selected) — managed
  photos via storage.get, external-mode unchanged.
- protectedImages / secureImages / photoResolver — read via
  storage.get; resolvePhotoStorageKey returns the canonical key.
- watermarkService / watermarkGeneratorService — persistent watermarks
  via storage.put.
- fileWatcher — bails out with a clear log warning when STORAGE_BACKEND=s3
  (chokidar can't watch S3); auto-import lands via the S3 prefix walker
  introduced in the follow-up commit.
- expirationChecker — small touch (event.expired webhook fire from #327
  shipping in the next commit).

Migration tooling
- backend/scripts/migrate-storage.js — one-shot --dry-run capable script
  that walks photos.path, thumbnail_path, hero_path, watermark_path and
  events.archive_path/download_zip_path; streams local → S3; sha256
  size-match skip for idempotent re-run; failures CSV.

Presigned-URL "Download All" (#328 follow-up shipped in this commit)
- routes/gallery.js — when STORAGE_BACKEND=s3 + event.allow_presigned_download
  + downloads enabled + watermark NOT enabled, /download-all returns a
  302 redirect to a 5-minute presigned S3 URL. Per-event opt-in surface
  ships in the next commit's UI.

Tests
- backend/__tests__/integration/storageBackend.test.js — parametrized
  contract suite running against BOTH LocalFs AND MinIO (18 tests, both
  backends — 36 cases total).
- backend/__tests__/integration/imageProcessor.storage.test.js — same
  parametrized pattern for the image processor (10 tests × 2 backends).
- backend/__tests__/integration/backup-s3.test.js — bootstrap fix:
  drop the redundant initDb() (001_init handles it) and remove
  schema-drift in configureS3Backup (app_settings has no created_at
  anymore and the unique constraint is on setting_key alone, not
  composite). 0/12 → 7/12 (5 remaining are unrelated assertion drift).
- backend/src/services/photoResolver.js — mixed-source events (reference
  mode with managed-uploaded photos) now fall back to managed when
  external_relpath is missing instead of throwing.
- tests/e2e/s3-storage-roundtrip.spec.ts — Playwright spec that
  auto-skips against local backend; full upload → serve → delete
  round-trip when run against an S3-mode backend.

Server wiring (server.js)
- initStorage() called after database init, before rate limiters.
- This commit's diff also includes the webhook delivery worker startup
  and the S3 auto-importer startup. Those features ship in the next two
  commits — co-located here for one bisectable diff per file.

Docs + ops
- README §"Storage Backends" — capability matrix, switching playbook,
  IAM policy snippet, MinIO/R2/B2 examples.
- README §"Webhooks" — also added here (full diff bundled).
- .env.example — STORAGE_BACKEND + STORAGE_S3_* + STORAGE_AUTO_IMPORT
  documented; WEBHOOK_* added in the same diff.
- .gitignore — re-anchor the existing `storage/` rule to `/storage/`
  so backend/src/services/storage/ (the new abstraction code) is
  trackable. The runtime ./storage/ data dir stays ignored.

Out of scope for v1 (per the issue): presigned URLs for individual
photo display (always streamed for protection middleware), CDN
integration, hybrid hot/cold tiers, S3 → local migration, multi-bucket
per-event.

backend/server.js

@@ -533,6 +533,7 @@ app.use('/api/admin/events', require('./src/routes/adminEventRename'));
 app.use('/api/admin/users', require('./src/routes/adminUsers'));
 app.use('/api/admin/event-types', require('./src/routes/adminEventTypes'));
 app.use('/api/admin/api-tokens', require('./src/routes/adminApiTokens'));
+app.use('/api/admin/webhooks', require('./src/routes/adminWebhooks'));
 // Public v1 API for n8n / external integrations (#322). Mounted under
 // /api/v1; auth handled per-route via apiTokenAuth (Bearer tokens).
 app.use('/api/v1', require('./src/routes/v1/events'));
@@ -601,6 +602,10 @@ async function startServer() {
     // Initialize database
     await initializeDatabase();
 
+    // Initialize storage backend (local fs or S3) — fail fast on misconfig
+    const { initStorage } = require('./src/services/storage');
+    await initStorage();
+
     // Initialize rate limiters after database is ready
     await initializeRateLimiters();
     logger.info('Rate limiters initialized with database configuration');
@@ -627,6 +632,16 @@ async function startServer() {
     await initializeTransporter();
     startEmailQueueProcessor();
     
+    // Start webhook delivery worker (#327)
+    const { startWebhookDeliveryWorker } = require('./src/services/webhookDeliveryWorker');
+    startWebhookDeliveryWorker();
+
+    // Start S3 auto-importer (#328 follow-up). No-op when STORAGE_AUTO_IMPORT
+    // is unset OR STORAGE_BACKEND=local — replaces the chokidar watcher
+    // for S3-mode deployments that drop files into the bucket directly.
+    const { startS3AutoImporter } = require('./src/services/s3AutoImporter');
+    startS3AutoImporter();
+
     // Start backup service
     await startBackupService();