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/src/services/watermarkGeneratorService.js

@@ -8,10 +8,10 @@
  * - Tracking regeneration progress
  */
 
-const path = require('path');
 const { db } = require('../database/db');
 const watermarkService = require('./watermarkService');
-const { getStoragePath } = require('../config/storage');
+const { resolvePhotoStorageKey, resolvePhotoFilePath } = require('./photoResolver');
+const { withLocalCopy } = require('./imageProcessor');
 
 class WatermarkGeneratorService {
   constructor() {
@@ -57,14 +57,15 @@ class WatermarkGeneratorService {
         return { success: false, error: 'Watermarking is disabled' };
       }
 
-      // Resolve the original file path
-      const originalPath = this.resolvePhotoPath(photo);
-      if (!originalPath) {
-        return { success: false, error: 'Could not resolve photo path' };
-      }
-
-      // Generate and save watermark
-      const result = await watermarkService.generateAndSaveWatermark(photo, originalPath, settings);
+      // Resolve the source via the storage backend (managed) or local disk
+      // (external reference mode). watermarkService needs a local file path.
+      const event = { slug: photo.slug, source_mode: photo.source_mode, external_path: photo.external_path };
+      const storageKey = resolvePhotoStorageKey(event, photo);
+      const result = storageKey
+        ? await withLocalCopy(storageKey, (lp) =>
+            watermarkService.generateAndSaveWatermark(photo, lp, settings)
+          )
+        : await watermarkService.generateAndSaveWatermark(photo, resolvePhotoFilePath(event, photo), settings);
 
       if (result.success) {
         // Update database with watermark path
@@ -83,31 +84,6 @@ class WatermarkGeneratorService {
     }
   }
 
-  /**
-   * Resolve the full file path for a photo
-   */
-  resolvePhotoPath(photo) {
-    const storagePath = getStoragePath();
-
-    // Handle external/reference mode
-    if (photo.source_mode === 'reference' && photo.external_relpath) {
-      const externalRoot = process.env.EXTERNAL_MEDIA_PATH || path.join(storagePath, 'external');
-      return path.join(externalRoot, photo.external_path || '', photo.external_relpath);
-    }
-
-    // Standard managed mode
-    if (photo.file_path) {
-      // file_path might be absolute or relative
-      if (path.isAbsolute(photo.file_path)) {
-        return photo.file_path;
-      }
-      return path.join(storagePath, photo.file_path);
-    }
-
-    // Fallback to constructing path from slug and filename
-    return path.join(storagePath, 'events', 'active', photo.slug, photo.filename);
-  }
-
   /**
    * Generate watermarks for all photos in an event
    * @param {number} eventId - The event ID
@@ -189,12 +165,13 @@ class WatermarkGeneratorService {
    */
   async processPhotoWatermark(photo, settings) {
     try {
-      const originalPath = this.resolvePhotoPath(photo);
-      if (!originalPath) {
-        return { success: false, photoId: photo.id, error: 'Could not resolve path' };
-      }
-
-      const result = await watermarkService.generateAndSaveWatermark(photo, originalPath, settings);
+      const event = { slug: photo.slug, source_mode: photo.source_mode, external_path: photo.external_path };
+      const storageKey = resolvePhotoStorageKey(event, photo);
+      const result = storageKey
+        ? await withLocalCopy(storageKey, (lp) =>
+            watermarkService.generateAndSaveWatermark(photo, lp, settings)
+          )
+        : await watermarkService.generateAndSaveWatermark(photo, resolvePhotoFilePath(event, photo), settings);
 
       if (result.success) {
         await db('photos')