fix: restore aspect-ratio layouts and improve hero image quality (#180)

- Fix masonry/mosaic layout regression where tiles displayed uniform heights
  instead of respecting image aspect ratios. Changed from fixed 150-500px
  height constraints to dynamic constraints based on column width.

- Add hero image optimization pipeline generating 1920x1080 images for
  full-width hero sections instead of using low-quality thumbnails.

- New /hero/:photoId endpoint serves optimized hero images with watermark
  support and automatic generation/caching.

- Add hero_url field to photos API response for frontend consumption.

- Migration 069 adds hero_path column to photos table.

backend/migrations/core/069_add_hero_path.js

@@ -0,0 +1,22 @@
+/**
+ * Migration 069: Add hero image path to photos table
+ * - photos.hero_path: path to hero-optimized image (1920x1080) for gallery headers
+ */
+
+const { addColumnIfNotExists } = require('../helpers');
+
+exports.up = async function(knex) {
+  console.log('Running migration: 069_add_hero_path');
+
+  // photos.hero_path (nullable - path to hero-optimized image)
+  await addColumnIfNotExists(knex, 'photos', 'hero_path', (table) => {
+    table.string('hero_path', 512);
+  });
+
+  console.log('Migration 069_add_hero_path completed');
+};
+
+exports.down = async function(knex) {
+  console.log('Rollback: 069_add_hero_path');
+  // Keep columns (safe rollback not removing data). Intentionally no-op.
+};