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.

frontend/src/types/index.ts

@@ -71,6 +71,7 @@ export interface Photo {
   filename: string;
   url: string;
   thumbnail_url?: string;
+  hero_url?: string; // Hero-optimized image URL (1920x1080) for full-width hero sections
   secure_url_template?: string;
   download_url_template?: string;
   requires_token?: boolean;