Restore thumbnail generation with Storj caching

- Restored thumbnail functions in src/thumbnail.rs with WebP support
- Added Storj caching for thumbnails (cache_thumbnail_to_storj, load_cached_thumbnail_from_storj)
- Updated handlers to use thumbnail generation with Storj caching
- Added ETag caching with MD5 hashes for optimal performance
- Updated documentation to reflect restored thumbnail functionality
- Removed local file caching in favor of reliable Storj storage

💋 Self-contained: Quoter now handles everything - uploads, thumbnails, and serving.

docs/features.md

@@ -1,6 +1,6 @@
 # Quoter Features
 
-Simple file upload/download proxy with user quotas and S3 storage.
+Simple file upload/download proxy with thumbnail generation and S3 storage.
 
 ## What Quoter Does
 
@@ -13,22 +13,29 @@ Simple file upload/download proxy with user quotas and S3 storage.
 
 ### 📁 File Storage  
 - **S3-compatible storage** (Storj primary, AWS fallback)
-- **Redis caching** for file metadata and quotas
+- **Redis caching** for file metadata and quotas (graceful fallback)
 - **Multi-cloud support** with automatic migration
 
+### 🖼️ Thumbnail Generation
+- **On-demand WebP thumbnails** with configurable dimensions
+- **Storj caching** for generated thumbnails
+- **Smart fallback** to original images if generation fails
+- **ETag caching** for optimal performance
+
 ### 🌐 File Serving
 - **Direct file access** via filename
+- **Thumbnail access** via `filename_width.ext` pattern
 - **Fast response** optimized for Vercel Edge caching
 - **CORS whitelist** for secure access (includes Vercel domains)
 - **Vercel-compatible headers** for optimal edge caching
 
 ## 🚀 Modern Architecture
 
-**Quoter**: Simple file upload/download + S3 storage  
-**Vercel**: Smart thumbnails + optimization + global CDN
+**Quoter**: File upload/download + thumbnail generation + S3 storage  
+**Vercel**: Global CDN + edge optimization
 
-💋 **Ultra-simple**: Quoter just handles raw files. That's it.
-💋 **Simplified**: Focus on what each service does best.
+💋 **Self-contained**: Quoter handles everything - uploads, thumbnails, and serving.
+💋 **Reliable**: No external dependencies for core functionality.
 
 ## Technical Stack
 
@@ -44,5 +51,7 @@ Simple file upload/download proxy with user quotas and S3 storage.
 - ✅ S3 storage integration  
 - ✅ JWT authentication
 - ✅ Rate limiting & security
+- ✅ Thumbnail generation with Storj caching
+- ✅ ETag caching for performance
 - ✅ Full test coverage
 - 🚀 Production ready