feat: add image optimization and settings page

- Add persistent disk cache for processed images
- Optimize image processing with short-circuit and quality settings
- Add WebP lossy encoding with configurable quality
- Add settings API endpoints (GET/POST /settings, cache management)
- Add database table for app configuration
- Add /settings page in backoffice for image/cache/limits config
- Add cache stats and clear functionality
- Update navigation with settings link

apps/api/Cargo.toml

@@ -31,3 +31,4 @@ uuid.workspace = true
 zip = { version = "2.2", default-features = false, features = ["deflate"] }
 utoipa.workspace = true
 utoipa-swagger-ui = { workspace = true, features = ["axum"] }
+webp = "0.3"