perf: resize images server-side before sending to Claude vision

- Resize to 1568px max (Claude's efficient ceiling) via Pillow
- Convert PNG screenshots to JPEG q85 (~5MB → ~200KB typical)
- Cap at 3 images per message (~4,800 token budget max)
- Graceful fallback if Pillow unavailable (Claude auto-resizes)
- Add Pillow + libjpeg/zlib deps to requirements + Dockerfile

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/requirements.txt

@@ -54,3 +54,6 @@ apscheduler>=3.10.4
 
 # Object Storage
 boto3>=1.34.0
+
+# Image processing (vision upload resize)
+Pillow>=10.0.0