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/migrations/core/061_add_watermark_path.js

@@ -0,0 +1,28 @@
+/**
+ * Migration 061: Add pre-generated watermark path to photos table
+ * - photos.watermark_path: path to pre-generated watermarked image
+ * - photos.watermark_generated_at: timestamp of watermark generation
+ */
+
+const { addColumnIfNotExists } = require('../helpers');
+
+exports.up = async function(knex) {
+  console.log('Running migration: 061_add_watermark_path');
+
+  // photos.watermark_path (nullable - path to pre-generated watermarked image)
+  await addColumnIfNotExists(knex, 'photos', 'watermark_path', (table) => {
+    table.string('watermark_path', 512);
+  });
+
+  // photos.watermark_generated_at (nullable - when watermark was last generated)
+  await addColumnIfNotExists(knex, 'photos', 'watermark_generated_at', (table) => {
+    table.timestamp('watermark_generated_at');
+  });
+
+  console.log('Migration 061_add_watermark_path completed');
+};
+
+exports.down = async function(knex) {
+  console.log('Rollback: 061_add_watermark_path');
+  // Keep columns (safe rollback not removing data). Intentionally no-op.
+};