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/watermarkService.js

@@ -2,7 +2,7 @@ const sharp = require('sharp');
 const path = require('path');
 const fs = require('fs').promises;
 const { db } = require('../database/db');
-const { getStoragePath } = require('../config/storage');
+const { getStorage } = require('./storage');
 
 class WatermarkService {
   constructor() {
@@ -234,19 +234,6 @@ class WatermarkService {
     this.cache.clear();
   }
 
-  /**
-   * Get the watermarks directory path, creating it if needed
-   */
-  async getWatermarksDir() {
-    const watermarksDir = path.join(getStoragePath(), 'watermarks');
-    try {
-      await fs.access(watermarksDir);
-    } catch {
-      await fs.mkdir(watermarksDir, { recursive: true });
-    }
-    return watermarksDir;
-  }
-
   /**
    * Get the file extension from a filename
    */
@@ -258,46 +245,42 @@ class WatermarkService {
   }
 
   /**
-   * Generate watermarked version of a photo and save to disk
+   * Generate watermarked version of a photo and persist it through the
+   * storage backend. The source must be a local filesystem path because
+   * sharp doesn't take streams; callers in S3 mode should materialize a
+   * tmp local copy via imageProcessor.withLocalCopy first.
+   *
    * @param {Object} photo - Photo object with id, filename, and path info
-   * @param {string} originalPath - Full path to the original image file
+   * @param {string} originalPath - Local path to the original image file
    * @param {Object} settings - Watermark settings (optional, will fetch if not provided)
    * @returns {Object} { success, watermarkPath, error }
    */
   async generateAndSaveWatermark(photo, originalPath, settings = null) {
     try {
-      // Get settings if not provided
       if (!settings) {
         settings = await this.getWatermarkSettings();
       }
 
-      // If watermarking is disabled, return early
       if (!settings || !settings.enabled) {
         return { success: false, watermarkPath: null, error: 'Watermarking is disabled' };
       }
 
-      // Verify original file exists
       try {
         await fs.access(originalPath);
       } catch {
         return { success: false, watermarkPath: null, error: 'Original file not found' };
       }
 
-      // Generate watermarked buffer using existing method
       const watermarkedBuffer = await this.applyWatermark(originalPath, settings);
 
-      // Determine output path
-      const watermarksDir = await this.getWatermarksDir();
       const ext = this.getFileExtension(photo.filename);
       const outputFilename = `${photo.id}_watermarked${ext}`;
-      const outputPath = path.join(watermarksDir, outputFilename);
-
-      // Write the watermarked image to disk
-      await fs.writeFile(outputPath, watermarkedBuffer);
-
-      // Return relative path for database storage
       const relativePath = `watermarks/${outputFilename}`;
 
+      await getStorage().put(relativePath, watermarkedBuffer, {
+        contentType: ext === '.png' ? 'image/png' : ext === '.webp' ? 'image/webp' : 'image/jpeg',
+      });
+
       return {
         success: true,
         watermarkPath: relativePath,
@@ -314,22 +297,18 @@ class WatermarkService {
   }
 
   /**
-   * Delete a pre-generated watermark file
-   * @param {string} watermarkPath - Relative path to the watermark file
-   * @returns {boolean} - True if deleted successfully
+   * Delete a pre-generated watermark file from the storage backend.
+   * @param {string} watermarkPath - Relative storage key (e.g. "watermarks/123_watermarked.jpg")
+   * @returns {boolean} - True if a delete was attempted (no-op if missing)
    */
   async deleteWatermarkFile(watermarkPath) {
     if (!watermarkPath) return false;
 
     try {
-      const fullPath = path.join(getStoragePath(), watermarkPath);
-      await fs.unlink(fullPath);
+      await getStorage().delete(watermarkPath);
       return true;
     } catch (error) {
-      // File might not exist, which is fine
-      if (error.code !== 'ENOENT') {
-        console.error('Error deleting watermark file:', error);
-      }
+      console.error('Error deleting watermark file:', error);
       return false;
     }
   }