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/Dockerfile

@@ -10,6 +10,8 @@ RUN apt-get update && apt-get install -y \
     libcairo2-dev \
     libgdk-pixbuf-2.0-dev \
     libffi-dev \
+    libjpeg-dev \
+    zlib1g-dev \
     && rm -rf /var/lib/apt/lists/*
 
 # Install Python dependencies