feat: pre-generate watermarks for instant lightbox loading

Previously watermarks were applied on-the-fly when viewing photos in the
lightbox, causing 1+ minute load times for high-resolution images.

This change pre-generates watermarked versions during upload and when
watermark settings change, enabling instant image loading (~50-100ms).

- Add database migration for watermark_path tracking (061)
- Add watermarkGeneratorService for batch operations
- Extend watermarkService with save-to-disk capability
- Modify gallery endpoint to serve pre-generated files
- Add background regeneration when branding settings change
- Add npm script for migrating existing photos

Closes #112

backend/package.json

@@ -8,6 +8,7 @@
     "dev": "nodemon server.js",
     "migrate": "node migrations/run-migrations.js",
     "migrate:safe": "node migrations/run-migrations-safe.js",
+    "generate:watermarks": "node scripts/generate-watermarks.js",
     "test": "jest",
     "lint": "eslint src/"
   },