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

infra/migrations/0008_add_settings.sql

@@ -0,0 +1,11 @@
+CREATE TABLE IF NOT EXISTS app_settings (
+    key TEXT PRIMARY KEY,
+    value JSONB NOT NULL,
+    updated_at TIMESTAMPTZ DEFAULT CURRENT_TIMESTAMP
+);
+
+INSERT INTO app_settings (key, value) VALUES 
+    ('image_processing', '{"format": "webp", "quality": 85, "filter": "lanczos3", "max_width": 2160}'),
+    ('cache', '{"enabled": true, "directory": "/tmp/stripstream-image-cache", "max_size_mb": 10000}'),
+    ('limits', '{"concurrent_renders": 4, "timeout_seconds": 12, "rate_limit_per_second": 120}')
+ON CONFLICT DO NOTHING;