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.

.env.example

@@ -114,6 +114,85 @@ APP_STORAGE=./storage
 APP_DATA=./data
 LOGS=./logs
 
+# ─── Storage Backend ────────────────────────────────────────────────────────
+# PicPeak can store photos, thumbnails and archive zips on the local filesystem
+# (default) or on any S3-compatible object store (AWS S3, MinIO, Cloudflare R2,
+# Backblaze B2, Wasabi, DigitalOcean Spaces, …).
+#
+# STORAGE_BACKEND=local (default)
+#   Uses STORAGE_PATH on the local filesystem. Backwards compatible — every
+#   existing deployment keeps working unchanged.
+#
+# STORAGE_BACKEND=s3
+#   Reads STORAGE_S3_* below. Auto-import via the filesystem watcher is
+#   disabled in this mode (S3 has no inotify) — every photo must enter via the
+#   admin upload UI/API. Run `node backend/scripts/migrate-storage.js` to copy
+#   existing local content to S3 before flipping the env.
+#
+# STORAGE_BACKEND=local
+#
+# STORAGE_S3_BUCKET=picpeak
+# STORAGE_S3_REGION=us-east-1
+# STORAGE_S3_ACCESS_KEY=AKIAxxxxxxxxxxxxxxxx
+# STORAGE_S3_SECRET_KEY=xxxxxxxxxxxxxxxxxxxxxxxx
+# Custom endpoint — set this for MinIO / R2 / B2 / Spaces. Leave unset for AWS.
+# STORAGE_S3_ENDPOINT=https://s3.us-west-002.backblazeb2.com
+# Optional namespace prefix inside the bucket — useful for multi-deployment buckets.
+# STORAGE_S3_PREFIX=picpeak
+# STORAGE_S3_FORCE_PATH_STYLE=false   # MinIO needs true; auto-on when endpoint is set
+# STORAGE_S3_SSL=true
+#
+# Minimum IAM policy (AWS S3) for the bucket above:
+#   {
+#     "Version": "2012-10-17",
+#     "Statement": [{
+#       "Effect": "Allow",
+#       "Action": [
+#         "s3:GetObject", "s3:PutObject", "s3:DeleteObject",
+#         "s3:ListBucket", "s3:GetBucketLocation"
+#       ],
+#       "Resource": [
+#         "arn:aws:s3:::picpeak",
+#         "arn:aws:s3:::picpeak/*"
+#       ]
+#     }]
+#   }
+#
+# EXTERNAL_MEDIA_ROOT (above) always lives on the local filesystem regardless
+# of STORAGE_BACKEND — reference-mode galleries are not migrated to S3 in v1.
+
+# ─── Outbound Webhooks (#327) ────────────────────────────────────────────────
+# PicPeak POSTs event/photo lifecycle notifications to URLs you configure
+# under Settings → Webhooks. Each delivery is signed HMAC-SHA256 with a
+# per-webhook secret in the X-PicPeak-Signature header.
+#
+# WEBHOOK_ALLOW_PRIVATE_URLS (default: false)
+#   Block URLs resolving to private IPs / loopback / .local etc. as an
+#   SSRF mitigation. Set to "true" ONLY in dev when your receiver is on
+#   the same docker network or localhost. Production deployments must
+#   leave this OFF.
+# WEBHOOK_ALLOW_PRIVATE_URLS=false
+#
+# WEBHOOK_DELIVERY_INTERVAL_MS (default: 5000)
+#   How often the worker polls webhook_deliveries for pending rows.
+# WEBHOOK_DELIVERY_INTERVAL_MS=5000
+#
+# WEBHOOK_DELIVERY_CONCURRENCY (default: 5)
+#   Maximum in-flight deliveries per worker tick. One slow consumer can
+#   monopolize all 5 slots — bump this if your receivers are slow OR ship
+#   a separate webhook-only deployment.
+# WEBHOOK_DELIVERY_CONCURRENCY=5
+#
+# WEBHOOK_HTTP_TIMEOUT_MS (default: 10000)
+#   Per-request timeout. Beyond this, the delivery is recorded as a
+#   network error and retried.
+# WEBHOOK_HTTP_TIMEOUT_MS=10000
+#
+# WEBHOOK_MAX_ATTEMPTS (default: 5)
+#   Total attempts before a delivery is marked failed. Backoff between
+#   attempts is exponential: 1m, 5m, 30m, 2h, 12h.
+# WEBHOOK_MAX_ATTEMPTS=5
+
 # Note on FRONTEND_API_URL (documentation only):
 # When using pre-built frontend images, runtime env vars cannot override the built JS.
 # Do NOT rely on FRONTEND_API_URL in Compose. Instead, keep VITE_API_URL=/api and