Build screenshot upload API for cc-web

## Problem When running Claude Code on cc-web (Hetzner), there's no way to share local screenshots. Need a lightweight upload endpoint. ## Approach - Small FastAPI/Flask script on cc-web (~20 lines) - `POST /upload` → saves to `~/screenshots/{timestamp}.png` - Auth: API key header or Tailscale IP restriction - Local alias: `ss-send` curls the latest screenshot to the endpoint ## Acceptance Criteria - [ ] API running on cc-web, accessible from local machine - [ ] Local bash alias for one-command upload - [ ] Auth in place (API key or IP restriction) - [ ] Claude Code can read uploaded images via Read tool