Update TODO.md with completed image processing items

TODO.md

@@ -41,15 +41,15 @@ A single linear checklist of tasks to implement the complete pixa caching image
 - [ ] Implement cache size management/eviction
 
 ## Image Processing
-- [ ] Select and integrate image processing library (libvips bindings or pure Go)
+- [x] Select and integrate image processing library (libvips bindings or pure Go)
-- [ ] Implement image decoding (JPEG, PNG, WebP, GIF, AVIF)
+- [x] Implement image decoding (JPEG, PNG, WebP, GIF, AVIF)
-- [ ] Implement image resizing with size options (WxH, 0x0, orig)
+- [x] Implement image resizing with size options (WxH, 0x0, orig)
-- [ ] Implement format conversion (JPEG, PNG, WebP, AVIF)
+- [x] Implement format conversion (JPEG, PNG, WebP, AVIF)
-- [ ] Implement quality parameter support
+- [x] Implement quality parameter support
 - [ ] Implement max input dimensions validation
 - [ ] Implement max output dimensions validation
 - [ ] Implement EXIF/metadata stripping
-- [ ] Implement fit modes (cover, contain, fill, inside, outside)
+- [x] Implement fit modes (cover, contain, fill, inside, outside)
 
 ## Security
 - [ ] Implement path traversal prevention
@@ -62,18 +62,18 @@ A single linear checklist of tasks to implement the complete pixa caching image
 - [ ] Add rate limiting global concurrent fetches
 
 ## HTTP Response Handling
-- [ ] Implement proper Cache-Control headers
+- [x] Implement proper Cache-Control headers
 - [ ] Implement ETag generation and validation
 - [ ] Implement Last-Modified headers
 - [ ] Implement conditional requests (If-None-Match, If-Modified-Since)
 - [ ] Implement HEAD request support
 - [ ] Implement Vary header for content negotiation
-- [ ] Implement X-Pixa-Cache debug header (HIT/MISS/STALE)
+- [x] Implement X-Pixa-Cache debug header (HIT/MISS/STALE)
 - [ ] Implement X-Request-ID propagation
-- [ ] Implement proper error response format (JSON)
+- [x] Implement proper error response format (JSON)
 
 ## Additional Endpoints
-- [ ] Implement robots.txt endpoint
+- [x] Implement robots.txt endpoint
 - [ ] Implement metrics endpoint with auth
 - [ ] Implement auto-format selection (format=auto based on Accept header)
 
@@ -88,9 +88,10 @@ A single linear checklist of tasks to implement the complete pixa caching image
 - [ ] Implement Sentry error reporting (optional)
 - [ ] Add comprehensive request logging
 - [ ] Add performance metrics (Prometheus)
-- [ ] Write unit tests for URL parsing
+- [x] Write unit tests for URL parsing
-- [ ] Write unit tests for signature generation/verification
+- [x] Write unit tests for signature generation/verification
-- [ ] Write unit tests for cache operations
+- [x] Write unit tests for cache operations
+- [x] Write unit tests for image processing
 - [ ] Write integration tests for image proxy flow
 - [ ] Write load tests to verify 1-5k req/s target